When I think about writing a review about Mass Effect, I find it hard not to be biased, because I really liked the setting. Here I'll try to be straight to the point so I won't drift away from topic and into my random fanboy ramblings.
Mass Effect 2 is THE sequel, storywise. Honestly, comparing past RPG sequels, I haven't felt the hook of nostalgia used as much as this game. When you venture throughout the galaxy, you will run into old NPCs, some important, some not, to catch up with. You will meet your old party members, and thankfully, Bioware is smart enough to give them significant roles to play in your quest.
Mordin might be noisy to some I think, not to me though.
But it's really the way they take Mass Effect into something like a movie, with huge focus on the cutscenes. Most of the dialogues, voice acting and cutscenes were choreographed so well, it seems like you are playing a movie at times. It gives exaggeration to a situation, making mundane chat into something that's seems cool enough to make players think that it's important.
The story continuation is just the way I like it. You saved the galaxy in ME1, but it's not all fine and dandy from there. You restored galactic peace but things don't always go so well. It really connects with realism, so straight away, ME2 pulls the player into the game.
I mentioned before that a good game, especially an RPG, is good only when you manage to immerse the player enough that he does not start to look at your game-flow structure. I didn't really think about that in ME2. All I wanted was to travel, meet random NPCs, look for interesting dialogues, and talking to my party members. In other words, I just want to explore.
Tali joins you!
Now on to the mechanics:
They only changed one thing in the FPS fights, that they resort to using ammos instead of plain overheating. I don't really know what's the rationale behind that, possibly to limit players from spamming pistol or sniper shots (which were the only weapons you can actually run out of ammo imo). Heh with this concept, suddenly I thought about how alike it is to the Nanoha anime.
Levels have been shortened from 50 to 30. Your skill points are now compressed and easier to understand. You are given a set of powers you can put points into and the more you level up a power, the more powerful it becomes and the more expensive the next upgrade is. At the highest level of each power (which is 4), you get to choose 2 different ways of the power to become (like having more damage or more AOE).
As I mentioned, there aren't weapon shops anymore. You just upgrade your weapons if you have enough materials for a certain upgrade. You can still find different weapons throughout the game, but just keep in mind that it's not like ME1 where you can just buy stuff.
As for character sidequests, needless you say it was very well done (when I say well done, I meant the whole package from storyline to voice acting to cutscenes), espacially Mordin's and Tali's. The rest were at least mildly interesting. I'm stating the sidequests here because I think it is worth stating in any review.
Garrus, the whimp to a badass.
Finally we have a new traveling system. I'm not going to elaborate much about moving your ship around the galaxy because it's self explanatory when you play it. Exploring of planets though, is alot less painful than before. Remember the old vehicle system and the randomized terrain you have to deal with? It's not there anymore thankfully. All you need to do is scan around the planet and shoot probes where you detect rare minerals.
Remember how those minerals are useless in ME1? They are now used for all your upgrades in ME2 so scanning planets become a bread and butter sort of thing. It gets tedious and occasionally boring but hey, it's way better than riding the stupid APC.
ME2 is truly worth a play. Apparently, I give up hope on Bioware too quickly when they released Dragon Age. ME2 could easily top my list of RPGs for the last decade I think and I really enjoyed it from the start to the end. It was the first time in many years when I forced myself to play a game.
Now if you'd excuse me, I'll go start my 2nd playthrough :)