How do you describe a game that has an almost limitless amount of story arcs? You don’t. Well, I could, but that would ruin the experience for you, and with Mass Effect 2 that is something you do not do. Even if you did tell someone the plot, chances are that they would not even experience half the things you did.
Carrying on where you left off with your original Mass Effect character is a unique experience; possibly creating the one true sequel in gaming. Now I know this is a big claim, but it really is special. In most sequels you are continuing the story laid in place by the game designers, but now you are continuing your own story. Think about that. Every decision you made in the original game is carried across into this one, with even more big decisions awaiting you throughout.
Very few developers have managed to lay down such huge moral decisions like BioWare have. You have the paragon and renegade options that were laid in place in the original. Although now BioWare have added a new way of making decisions: during cut-scenes an option will flash up on the screen, pressing the corresponding button will trigger an action, either renegade or paragon. This adds a new dynamic edge to the moral system, forcing you to think quickly.
A lot of the moral decisions come from your squad, or more so their stories. Throughout the game you have to recruit squad members and gain their trust, in doing so you become emotionally attached to them, caring about them more, or maybe even hating them more. The choice is yours. I have never played a game that has moved me like Mass Effect 2. You have to play it honestly, ignoring possible achievements, to really get the most out of the game.