"Epic" doesn't begin to describe this absolutely terrific game.
It doesn't take long for the s**t to hit the fan in ME2. Literally 5 minutes into the game, you [Commander Shepard] die. Yes, you die. Sowhat good is a game where you die 5 minutes into it? Well, that's just a little of what's to come. 2 years -- and billions of space credits later -- you are being yelled at by a woman by the name of Miranda to get out of bed, because the station your on is under attack. That's where epicness starts to build up; you can customize Shepard's appearance anyway you want to b4 he comes back to life. And when you do wake up and start getting told to grab a gun and start shooting mechs, you'll feel how smooth and precise the gunplay/cover/combat mechanics are. Anyways, you escape the station and are told to talk to the Illusive Man.
Another epic feature about this game is the conversation feature, which allows you to mold the attitude of Commander Shepard anyway you want to, similar to that of the first ME. Wanna be the hero or badass? That's all up to you, and neither is worse than the other, though the interrupt system is totally badass (you'll know what I mean when you play). However just because you can implement your choices in the game, you still should play smart and make smart decisions, because those decisions can seriously mean the differnce between success of your mission and failure (and can also affect your relationship with your crew, which can be vital if your wanting to find some love with a crew member, preferably Miranda ;]) Anyways, throw the combat and conversation elements together and you've got alot of meat on the bones of ME2.
But one element that adds even more meat to the bones is exploration and gameplay. Besides some unskippable missions with the Collectors, you can pretty much go wherever you want, whenever you want and explore the galaxy and planets to your hearts desire. And boy, is the galaxy HUGE. Exploiring all of it will add numerous hours to your playtime, as if the combat/conversation mechanics weren't enough. Even if you do complete the "suicide mission" (because you can die in it), the game doesn't end, so you can continue to play through and explore, and also so you can import your character into Mass Effect 3, whenever it comes out.
So, simply put, throw together some excellent gameplay, gunplay, RPG elements and exploration and you've got one of the most epic games of all-time Mass Effect 2. Even though it doesn't have any multiplayer modes (which is a shame =() you'll give a lot of your life away to Mass Effect 2, which honestly, isn't such a bad thing. "Epic" doesn't begin to describe this absolutely terrific game.