One of the coolest things about Mass Effect 2 is your ability to make moral choices which affect how people react to you within the game. As you amass good (paragon) or evil (renegade) alignment points, you unlock different choices in the dialogue and cutscene scenarios of the game.
This adds depth, realism, and a new layer of interest to the story. You feel as though you're really guiding your character and shaping them as you progress through each quest.
In a sense, this moral alignment system also makes the game feel more cinematic. Your character, Commander Shepard, takes on a personality of his or her own. You can play a character of mixed alignment, as well, and simply determine your moral choices based on each situation, rather than on any set persona your character might have that you've determined ahead of time.
While there are a lot of interesting facets to this game - the class variety, bioitics, weapons, etc. - the moral alignment system appeals to RPG fans because of how differently you can make your character and each Mass Effect 2 play-through turn out.
In a sense, you can play this game a thousand times and unlock all kinds of new dialogue choices because the moral system gives you the freedom to choose how the world reacts to you.
You don't have to be good or bad in ME 2; you can choose a neutral path, playing a Shepard who isn't necessarily tough or peaceful. In actuality, it feels less like you're determining good or bad, less like the cut and dried, black and white moral choices. It's more whether you deal with problems with brute force and intimidation, or choose peaceful outcomes as often as you can.
But again, having the opportunity to play mixed alignments really makes the game a lot of fun, especially since you'll end up unlocking new dialogue trees for the different characters you meet. Since ME 2 is such a dialogue, character-driven game, this is especially important.
So if you end up buying Mass Effect 2, make sure you experiment with the different dialogue trees and such. If you've already played the first game in the series, you can important your character and continue progressing the story.
When Mass Effect 3 comes out, you'll get to experience even more cool dialogue and moral choices, since you'll be able to import your character there as well. The story will continue, and new and old characters will be returning for you to encounter.