Mass Effect 2 expands on everything that made the first game great.
What can I say about this sequel that hasn't already been said about its predecessor..? Just like the first title, Mass Effect 2 is a sci-fi RPG whose story depends heavily on the choices made by the player. But the manner in which these choices effect the unfolding of the plot far and away surpasses that of the first game. It makes the experience so much more personal.
The story begins more or less right where the first game left off: the Citadel is recovering from the rogue Spectre Saren Arterius and the 'Reaper' Sovereign's assault, and you, Commander Shepard, were assigned to eliminate any remnants of their army. On your journey you discover that humanity's existence is threatened by an enigmatic new threat. Upon further investigation, you learn that this threat is linked directly to the Reapers, and resolve to stop them by any means. What ensues is an odyssey that takes you to all corners of the galaxy to assemble a team to take the fight to this mysterious enemy.
As with the previous title, there are many interesting, often likable, characters with unique experiences, back stories, and personalities. These men and women will either help or hinder your progress, depending of course on how you choose to relate with these characters. These faces are new and old, so any friends you've made in the last game will likely turn up in this one. (Needless to say, anyone who died is, well… dead.) It's unfortunate that many newcomers to the franchise will likely not be familiar with the characters, and therefore won't know them as well as someone who has played the first game. (While the back story comic helps, it doesn't quite suffice.)
The sheer scale of this game is immense: on top of all the character development that BioWare has put into Mass Effect 2, there is so much more that can be done that doesn't necessarily pertain to the story. Case in point: explore the galaxy at your own leisure. Shop for items of all kinds, have a drink or two at a local bar. Search of precious minerals with which you can research upgrade projects for your ship, your weapons, and / or your armor. Heal your scars, obtain new deployable heavy weapons, install a new main gun and heavier armor on your ship; whatever's available it can be done.
I don't think my words can do this game justice, but put simply, Mass Effect 2 is a masterpiece with an immense in-game universe and similarly sized replay value. I expect great things from the next one.