The best sequel they could've made
It started right in the opening sequence. When I was playing it, a couple of my friends were sitting behind me and, being the Engineering nerds we are, the first thing we noticed was that, when in space, there really is no sound besides your breathing. It might seem an insignificant aspect, but Bioware really did play it well by using that little sound factor as a means to increase the desperate atmosphere.
I do had some minor gripes about the story in the beginning, namely with how far-fetched the whole Lazarus project is. But it is sci-fi and you do understand why the Lazarus Project was something only done once, and with you. I also had my doubts about how Mass Effect 2 would be importing key decisions from the original, but it worked. While not all of your former party members are back, you get two back in your squad and the others have brief cameos (of which my favorite was Wrex's). It bugged me at first, but then I understood that certain characters were not recruitable this time around because they would be needed in the third installment and not having them in your party was the only way of making sure they would not die. Plus, some (if not all) of the new members are so likable that I just didn't mind having them around instead of the old ones.
There's also the aspect of romances, something Bioware has almost always included in their games. It is something that is, indeed, secondary in a game, but whether you like it or not, romances increase both re-playability, role-playing and the veracity of the story. I tried them all and, surprisingly, the best ones are with aliens. I'm aware it was probably fanservice from Bioware's part, given the first game, but it was fanservice done right.
Still in terms of story and characters, another thing I had not believed at first was related to character death, particularly Shepard's. Bioware advertised the suicide mission and I thought it was just marketing because, in all my playthroughs, no one died on me. Thus, I decided to change things and screw around for a bit and I realized it was not marketing. Make bad decisions, mess around with your squad's loyalty, have someone in your squad at the wrong place and you, or them, or someone else will die. Your ending can go from heroic to disastrous with one move - and that, was brilliant.
Combat-wise, Bioware has stripped the game of almost all of its RPG elements and, while I had moaned my complaints about it prior to the game's release, after playing it three times, I'm glad they did. Combat is a whole lot more fun than it was in the original Mass Effect. Side missions are much more varied than in the original and planetary exploration, while still a bit dull and boring, is much better than with the Mako. Not to mention it's actually useful for upgrades instead of plain useless like it was in the first Mass Effect.
Basically, pretty much everything people complained for, Bioware re-did. It was Mako, the easy combat, the side-missions, the crappy inventory system... all of them have been re-done and, in my opinion, for the best. Even the graphical, loading and sound issues have been addressed to and, while the original Mass Effect had trouble running in my gaming PC, this one runs even on my four-year-old, only-capable-of-programming-for-College laptop.
All this topped with with a terrific script and superb voice acting - there is nary a weak link in the cast... And, surprisingly, I found the worst voice acting to be Mark Meer's, the male version of Shepard, who, when compared to the female's voice actor, Jennifer Hale, does a sub-par job. Martin Sheen stands out as the Illusive Man, and I found both Courtenay Taylor, as Jack, and Liz Sroka, as Tali, to have particularly engaging performances.
All in all, Mass Effect 2 is a fantastic game if you're a newcomer, and even more so if you've played the first, and a step taken in the right direction for the ambitious plans Bioware has for the trilogy. Even though I find myself a bit set up for disappointment when the third and final game comes out, a little part of me is hoping Bioware will outdo themselves with Mass Effect 3 as they did with Mass Effect 2. And I think they will.