Since I've wrote the review for the first game, I feel quite entitled to do the same for the second one. This will be more of a comparison, though, so if you haven't read the review of the first Mass Effect, you should do that first.
Graphics and controls
To start off, there are things that are better and things that are worse in ME2 compared to ME1. Unfortunately, there are more of the latter than the former.
First off, the graphics hasn't changed, it's as good as ever. Diverse environments, different atmospheres and other things like that are the same as in ME1, that is, superb. You can also see quite a lot of meshes from the previous game here, for example, most of the crates in the game are the same white chest-high crates you've seen in ME1, which is not bad since it's a kind of a reminder of the first game. Oh, and the Normandy-A, as I like to call it, is nicely done as well, now being white instead of black and way bigger. Reminds me of the Enterprise refit from Star Trek motion pictures, too.
The second thing you'll notice in the game are controls. And that's where things get bad. Gosh, the controls!.. This is the most painful part of Mass Effect 2. First off, they totally threw all the key setup from the first game out of the window. Now by default the tactical menu is accessible through left Shift, and the use key is Spacebar. In fact, it's not really the use key you're used to know from the first game. They threw pretty much everything to this one key! When you press it, it can do: a) Sprint, b) Use, c) Take cover and d) Leave cover. The holster weapon has been remapped from Q to H, too. Well, at least you can remap the keys, so I've reverted them back to what the original had, which makes it less painful. But still - due to the omnifunctional use button I had to bind both E and left Shift to the same button, which doesn't make much sense. To me, it feels quite a lot console-minded to have buttons like that. When I press a key, I want to know what it does and what it doesn't. Here, I often times find myself leaving cover instead of reloading the gun! Also, there is now a new button for melee combat, which I neither like nor dislike. Oh, and one more thing that is pretty obvious oversight is that when you change key bindings, all help texts still list old ones, although that could be fixed easily by using the functions to access the input INI file.
That brings us to the next thing - combat. Improved? I think not! First of all, you now have the so-called "thermal clips" which are explained as something that lets your guns not overheat. Well, in theory that would be very nice. In practise, you see that this story is not at all true because if you run out of clips - that's it, you're through. You'd expect that you would throw away the clips and engage manual mode that would work like in the first game, but nooo, you're just stuck there as if you didn't have ammo, despite the fact that the Codex explicitly states that all weapons have AI systems that can create ammo from any material! So not only is this illogical, but also makes battles a whole lot more inconvenient since you have to be on the look out for clips all the time instead of focusing on tactics. And why was the old system bad, anyway? It was innovative and worked just fine (with the exception of the rare "infinite overheat" bug).
Now, the tactical menu. Compared to ME1, it's just featureless! There you could instantly see the health, shields, powers, orders and weapons very quickly and easily. Here, it's been reduced to a small batch of icons that only show powers. In fact, there is no real indication of your party members' health or shields, only a small coloured icon next to them that gives little to no information. Your own health is shown only by the amount of redness on the screen and the sounds you hear, and your shields are represented by a half-circle that is hidden most of the time. (Edit: health is actually represented by another half-circle bar, after you get hit.) Also, the order commands are nowhere to be found - they have been replaced by key bindings (Q to order one member to go, E to other the other, C/V to get them back to follow you). Well, at least you can remap them and you can also use them when in the tactical menu.
About gameplay mechanics. There is no more omnigel and no more medigel, at least not in the usual sense. Medigel is now used to fuel the Unity power. You also use Unity to heal yourself, too, although it's only useful in the most critical situations, as regeneration can take care of everything otherwise. (Edit: That's only true on lower difficulties.) The good thing is that there is Shield Boost, in a form of a special power. You can choose a few versions of it, too - Biotic shield, Fortification, Geth Shield... But I'm not sure how they are different.
There are some interesting additions, though. There are more weapon types, and you usually carry around 4-5 at a time. You have one heavy weapon, which is a nice touch that helps a lot with taking out bosses. There are also things like sub-machine guns (which, oddly enough, have almost the same looks as the pistol), pistols are overall more useful now since you can headshot people. Now the power usage is also different - instead of them cooling down individually, they block every power, but regenerate a whole lot faster, which means that you can spam one power over and over again, which is amusing. But that also means that you will be powerless for some time if you heal your party members. That also usually makes fights a lot less varied, as there is little reason not to use the same power over and over again.
Now about inventory management. Or what's left of it. There is no more Inventory menu at all! That means that the whole process was heavily dumbed-down. You no longer get anything more than credits from lockers, you get new armour and new weapons only by researching them (and new ones always replace old ones instead of being alternative). You buy only research from stores, and you can't sell anything at all. So while this system makes you spend less time reviewing your inventory, it takes the whole excitement from getting new items and hoping that one of them could not only be better than you already have, but also look better (you always see the appearance of the item before you research it). The earlier weapon, ammo and armour upgrades are no more, too. Instead, using cryo or incendiary bullets is made into a power, and there are no more things like explosive ammo. You are also limited to using ammo that fits your class - Infiltrators get cryo and disruptor ammo, for example. There are also no more grenades, and although I didn't use them much in the first game anyway, I can't help but wonder why they discarded them entirely - it was a good way to clear out enemies around the corner or those that stand just in front of the doors. (Edit: enemies do use grenades, and DLCs allow you to use them too, but now they are the bouncy, timed type and thus not nearly as useful. They are also counted as a power.)
Talking about that, the shields system is way too complicated now. Earlier, everyone had shields and health. Now they have armour, shields, biotic barriers and health. The three terms there logically would mean the same thing (especially since in the Codex, it was already established that shields are barriers already, and that armour is a shield-generating array). But here they are all different, and even worse, they now protect from certain powers, and this makes said powers pretty useless against any stronger enemy. (Edit: actually, "shields" should be the only type of shields; "barrier" should be a power that strengthens your shields; and "armour" should mean damage reduction, not an extra health bar. Alas, they are all health bars in the game...)
Now about taking cover - it's way too difficult than it has to be, too. In the first game, it was easy - touch the wall and you'll take cover. Plain and simple. Here, however, you need to press a button, and what's worse is that only designated walls can be used for cover, which again doesn't make any sense. Though the sliding animation when you take cover and the fact that you can mount on nearly any chest-high wall you come across is a nice touch. (The keys, again, are horrible for that, though - to mount, you need to press the forward button and press the Use button, unlike in the first game, where only the use button was needed). And there is no more crouching by you. Your party members will crouch, but not you, you have to stand in the middle of the battlefield having the huge collision box.
Vehicles and galaxy map
The AI is better, however. In the first game, you take shelter just to see your party members standing out in the open just tempting every enemy to shoot. Here, if you take cover, your party members will do that too - not only that, but they will also engage the enemy within a larger radius from you, making snipers far more useful. Oh, and the party members don't die nearly as easily - probably because they're smarter. Another interesting thing is that if you point your gun at them, they will crouch so they wouldn't block your bullets like in the first game.
More about mechanics - the galaxy map navigation is more difficult now. You now control a miniature Normandy-A and have to fly to the planets instead of just clicking on them. Well, it looks nice, but it takes some time and you can't view the whole system easily. Furthermore, scanning is a whole lot different, and it almost makes me believe it was designed for use by batarian players, and not humans... You have a scanning reticle and have to look for unusual readings in a kind of a radio chart that changes depending on the position of the reticle. However, the chart is way to the right from the planet, so you'd have to have more than one pair of eyes to be able to monitor both, which leads to a whole lot of redundant swipes and missed resources. And that process is really too slow. You have to blindly wiggle the mouse to get any readings, then fine-tune it so you get the maximum resources, then launch a probe, and then repeat it around 10 times for every single planet. You'd think that an upgrade, that claims to give you high quality scanner data, would help with this, but it actually doesn't do almost anything. And you have to buy probes for credits. Normally it's not an issue, you scan a planet, you go to a refuel station, you buy the probes, you scan another planet. However, some systems just don't have a refuel station! That means that you have to use a mass relay to get to another system, then resupply, then get back, which is annoying. And using the FTL drive now costs fuel, too, that means double the work, credits paid and delay for you. Why would you do that, anyway, since someone else should take care of trivial things like that? Well, probably because there was nobody to replace Pressly... The only good thing is that a patch makes this easier by making the reticle around twice as big, and you're not forced to mine all the resources from every single planet (although that means that you will miss assignments).
Vehicle mechanics. They didn't fix the Mako - they outright dropped it! That's just wrong! Instead, you have a PeaceKeeper-style dropship that doesn't help you in any way (the story is that Normandy-A is too large to land so they need to send you via a shuttle, but why not just drop the Mako from orbit as in ME1, and then send a shuttle to take it back to the ship in orbit?..) That means that you no longer have that awesome vehicle and you have to fight the hard way every time. On the bright side, though, there are no more repetitive buildings here - everything is unique. But that means that you will be primarily involved in missions and not assignments, which would otherwise be a good way to relax & enjoy the view and the ride.
The Firewalker pack DLC, however, adds another vehicle to the arsenal - the Hammerhead. What's that vehicle like? If you know Unreal Tournament 3, imagine a Manta that can jump five times as high, has Scorpion's boost ability and fires Leviathans' homing rockets. That means it's overpowered as all hell. Well, it kinda makes up for it by being fragile and enemies shooting homing things at you as well. I have to say that it's enjoyable, though. You can reach a lot of places where Mako would be useless, and ramming into an unsuspecting geth with full speed and seeing it fall down the cliff is pure joy. There are two main downsides for this system, however: 1) you need to get the DLC first, and 2) there are only around 3 missions with the Hammerhead.
Sounds and Conclusion
The hacking has been changed, too. No more rotating reflex puzzles - this time there are two puzzles, one is a memory one where you need to match pictures to rewire a circuit board, and the second one is where you have to find matching pieces of UnrealScript code. The first one is quite easy, but the second one is hell because for one, all code pieces look very much alike, with the only difference being the colour and the positioning of the code; moreover, there are red blocks which cancel the hacking altogether if you manage to hit them; furthermore, there is a strict time limit and if you go over that, because of the lack of omnigel, you lose credits (from the locker) instead. Now combine that with the WASD-Space (or WASD-omnifunction button if you have the patch) controls to move the pointer and you have one puzzle that you won't like throughout the game. And time no longer stops when you are doing the puzzle, too, which means that if there is an enemy you missed, your hacking will be interrupted and you will never be able to recover the contents again!
There is also a change in the skill system. Now you have less options, which makes it easier to choose but at the same time makes the character less personalised. And in an RPG, I expect to be able to have all the control. There are overall a lot less skills to learn, too - there is no more Electronics, Charm or Intimidate etc.
One more thing that I feel is not right is the Citadel. In ME1, it represented the galactic community and the civilisation. There was always much to do there, much to see and much to learn. Much to earn, too. In ME2, it was dumbed down to restricting you to a single ward, and while you can theoretically go to the Presidium, all you see is the Earth embassy. It seems that they wanted ME2 to be all new, but some things should be left so you could see how things changed and the feeling of familiarity! What happened to Sha'ira? What happened to the Emporium keeper? To the Shadow Broker dealer? To Morlan? We will never know... Well, except for the letters they send you, but it's not the same. Similarly, you can no longer access the part of the galaxy you played in in ME1, save for the Local cluster which is not that interesting anyway. And it makes no sense, too, since the Codex states that the Primary Relay links to one and only one location, from which Secondary Relays have to be used. Here, we see the Primary Relay having two directions.
Now on to the story. It's as good as in the first game - brilliant character development, a lot of plot twists and interesting trivia that give the Mass Effect universe immersion. One thing that I like is that there are quite a lot of humorous references in the game, for example, advertisements show Hamlet in Elcor that was promised in ME1, or when you order a drink in the Wards, you ask "It's.. It's green?" (Star Trek TOS: "By Any Other Name" reference). ME1 didn't have that many nice references, although the radio station from the DLC "Bring Down the Sky" stood out as having a reference to Star Trek TOS: "For the World is Hollow and I Have Touched the Sky".
Oh, and your party: there are both old and new characters, and it's interesting that the choices you made in ME1 affects ME2. There is also one member of the party that I always wanted to have there, but I can't tell you more without going too deep into the realm of spoilers :)
And finally, the music and sounds. The sounds did get a very nice update in ME2. I complained about the lack of EFX in ME1 - well, it's there in ME2, and very well implemented! The audio overall feels more 3D as well, and hearing muffled sounds when you're badly wounded is a nice touch. Firing sounds are even more pronounced now, too - even pistols sound (and are) powerful. The music, however, is not very remarkable, save for the "mission complete" tune. Even the credits music is unremarkable, a striking contrast to ME1 credits theme.
In conclusion, if I was to choose a word to describe Mass Effect 2, it would be the word "dumbed-down". There are a lot of things that were removed or changed although that makes no sense even from the universe point of view, and some things are oversimplified (like the inventory) while others are too difficult than they should be (like scanning planets). They key bindings are horrible, too. But Mass Effect 2 does have the same good story, the same good graphics (even more diverse than before), more humour, improved team AI and way better sound that are always good. So while I still prefer the first one, the second one gets a good score of 8.5/10.