BioWare's greatest strength has always been its focus on telling a good story, its focus on delivering characters that are well-rounded, deep, full of history and things to tell. One of the many moments in Mass Effect 1 that propelled the game from really really good to pure bliss was the conversation you could have with Ashley, about her history, her sisters, parents, and life in the military. The crowning jewel of the conversation was an argument about her faith in god, and whether or not religion has a place in a world where mankind has gone to the stars. It made my jaw drop to the floor.
This very conversation embodies BioWare: they make you care about the characters in the game. They are flawed, just like humans in real life (even when they're aliens), BioWare games don't have perfect heroes or baby-eating bad guys; even Saren, one of the main villains in part one, was undeniably tragic, an unknowing victim of Sovereign's games. I felt sad after killing him, because I knew that it wasn't all his doing.
These are the kinds of things that make BioWare games great. It makes sure that whatever choices you make, even those that concern bad guys, you'll put your controller down to mull over the consequences they might have. While playing ME1, I regularly had to stand up and walk around to contemplate the choices I was about to make.
The new characters in ME2 fail to endear themselves. They lack the depth I have come to expect from characters in BioWare games, with the end result being that I just didn't care about them, with Legion being the only notable exception. It's not that they were badly characterised or something (this is still BioWare we're talking about), they just didn't... Make me care all that much about them.
It is very telling that the characters I cared about the most were the ones carried over from part one. Joker returns as the comedy note, and he does a great job at that (especially the verbal sparring between him and EDI). Tali'Zoriah is still her sweet little self, and Garrus was surprisingly interesting too (I found him a bit bland in ME1). You'll also run into a number of other ex-team members you can't recruit, most notably Ashley, Liara, and Wrex. All three of them appealed to me; especially Shepard's falling-out with Ashley was heartbreaking and frustrating, as nothing you could say or do could make things right again.
Overall though, while the characters weren't as good as the ones in ME1, the special missions you do for them (one for each to gain their loyalty and unlock bonus powers), were quite interesting. Jack's return to the facility she grew up in was memorable, and Mordin's confrontation with his work on the genophage really tested your morals (I'm still undecided about whether or not I support the genophage). The cherry here was Tali's mission back on the Flotilla, which really blew my socks off.
It's sad that the ME2 team failed to implement some sort of group dynamic. I mean, in Dragon Age you could upset multiple team members with your decisions, but none of that was present here. You'll run into a few scripted falling-outs (Tali/Legion and Jack/Miranda), but that's about it.
The loyalty system is pathetically shallow too; characters are either loyal, or they're not, and the effect it has on battle is minimal. I had hoped for a more intricate system. I mean, activating Legion should've pissed off Tali to no end; me helping Mordin should've affected Grunt. Characters live on their own little island, and they're either loyal, or not - I figured we'd see a more complicated system where completing a mission would affect the stats of your team mates, much like it did in Dragon Age.