A streamlined/simplified continuation of ME1 with improvements around the board and additional plot divergences
Despite these nitpicks, Mass Effect 2 is better than its predecessor in almost every aspect in my book. I didn't see how they'd improve the story of mass effect 1 (sequels have this tendency to be underwhelming compared to their originals) but ME2 really came through. The ending will be particularly exciting, but who knows what you'll get. The game's multiple ending bring back a neat feature that started with chrono trigger.
Gameplay: New things have been introduced to make combat more fun. There are now different types of shields that enemies may have, and you'll have to be dynamic to effectively destroy the enemy this time.
Graphics: Better optimized than ME1, and just outright sexy. ME2 looks great and the characters are all more detailed this time and have more realistic shading.
Sound: The music has been injected with more epicness this time around, and especially at the endgame the orchestral soundtrack will really get you in the mood to save the galaxy
The game is a hell of a journey and you'll walk out of it salivating at the thought of mass effect 3.