About a month ago I reviewed Mass Effect 2 - the very same afternoon I finished the game. After playing through it two more times, and analyzing it further, I have to say that it's one of the most brilliant games I've ever played, and my new current-gen favorite. It's hard to really talk about this game without exposing some huge spoilers, but I'll give it a shot.
First of all, they created an entire galaxy with this game. Within this galaxy exist a number of different races and cultures. Each have their own strengths and weaknesses, and relations with other races. In that sense, it isn't too much different than Star Trek. While the galaxy isn't perfectly united, most of the races generally live in peace. The main galactic government is known as the Council, made up of three races as of the start of Mass Effect (humanity isn't one of them). Humanity is relatively new to galactic civilization, and besides the main plot, the entire first game is about the human race establishing themselves.
In the second game however, the entire plot revolves around building a team to destroy a very powerful enemy. You gather some of the most dangerous individuals in the galaxy and take on what many consider to be a suicide mission. While it's possible for everyone to come out alive, it's also possible to get everyone on your team killed, including yourself. Every single one of these characters has a well developed personality and a strong back story to go with them. With the exception of several human squad-mates, every team member is of a different race. The game indirectly forces you to get to know these characters in order to make them loyal to you. Some of the characters seem unlikable at first, but after you start talking to them, you'll realize how well designed their characters are. The only character I didn't like was Zaeed (a mercenary), but he's optional downloadable trash anyway.
As I mentioned in the review, throughout the game you make tons of decisions. Do I kill this young criminal, or let her go? Do I threaten the looters, or do I be polite with them? Do I execute the hostage takers or reason with them? There are tons of decisions like this, and pretty much all of them affect the rest of the game somehow. Not only that, but if you load your save file from the first game, every choice you made in Mass Effect 1 affects Mass Effect 2 somehow. I'm still very impressed with how much weight your decisions have on your experience throughout the game, and everyone who plays it will have a different experience. This doesn't just make Mass Effect feel like a game, but it makes it feel like your game; you feel like you're Commander Shepard rather than just playing as him/her. According to IGN's review, Mass Effect 2 is the most personal game ever released, and I fully agree with that statement.
What really screams of these game's qualities however, is that even though both games are full of huge cinematic battles, fast-paced brawls and tons of beautifully crafted locations, what I remember the most are the quiet moments. I remember many of the conversations that Shepard has with his/her teammates. I remember all the different characters you either help out or galactic cultures you learn about. That's what truly makes this game a true masterpiece; how well written the dialogue is, how well done the voice acting is, and how real this galaxy feels when everything comes together. If you have an Xbox 360 or a capable PC, I cannot recommend this game enough.