BioWare is keen on making your decisions in Mass Effect 2 matter, so much so that you might not be able to import your saves into Mass Effect 3 at all.
BioWare wasn't exactly shy about the fact that Shepard could die in Mass Effect 2 and anyone who has played the game has probably seen loading messages saying that you can import your save in the next game, as long as Shepard survives. Well, it would appear that BioWare is playing hardball, because if Shepard is dead, your ME2 saves are useless.
Producer Casey Hudson spelled it out in black and white to Game Informer: "Dead is dead. Mass Effect 3, as with the rest of the trilogy, is Shepard's story. If you have a dead Shepard at the end of Mass Effect 2, that saved game won't import into Mass Effect 3. You can play Mass Effect 3 if you died in Mass Effect 2 of course, but you'll have to create a new Shepard. Harsh? Yes. But we wouldn't be serious about the concept of a suicide mission if you couldn't die and your death didn't have serious consequences."
It's not really a surprise that Shepard's survival is canonical, but it's an interesting way of adding weight to the decisions you make, and not one that every developer could pull off. I imagine that there are going to be quite a few people grudgingly playing through the game again, but given how good it is, that's no bad thing.