Everything you expect from a first class Action RPG. The programming glitches from ME1 have been totally fixed in ME2.
The story is interesting, the characters are believable and complex, the dialog is snappy, the graphics are top notch, the gameplay is interesting and direct...
If there's anything I would actually fault the game for it's that there are too many characters and too many paths. A conceit of current RPG design is that more story paths means a deeper and more rewarding game. However, it tends to dilute the focus of a story and distract the player with irrelevancies.
Ideally, there should be a strong central story arc that the player is more or less locked onto but allows tactical freedom for how objectives are obtained. The "choose your own story" system of "turn to page five if you want to shoot Jack" isn't as compelling as some seem to think.
This said, unlike so many games out there the writing in Mass Effect 2 is excellent. The characters even in their multitude are interesting. Most importantly however, the Illusive man, the central mystery character of the game is just about perfect.
All and all its' hard to complain about ME2. But from a story perspective it would have been better if they had spent more time on the central story and less time filling out a dozen irrelevant side stories about characters that though well written are irrelevant to the core plot. Cut out half the side characters and spend the balance on the core plot and it would have been perfect.