The various choices that impacted the ending of Mass Effect 2 were pretty complex, and now Mass Effect 3 is primed to amplify that feeling by tying together choices from all three games. BioWare's Casey Hudson explained how this will bring it all together in an interview with PC Gamer.
"I think a way to think about it is if you made decisions early on, you'll see them affecting this," he said. "And the decisions you might want to make that go against those prior things are gonna be harder."
Specifically, Hudson seems to imply that no matter what decisions you made in past games, it will be more difficult to change course now. "Killing the Rachni might present opportunities in Mass Effect 3 that you wouldn't otherwise have, but if you don't take those opportunities and you try to do something in opposition to that, then it would be harder for you than if you work with it. Similarly with the decisions at the end of Mass Effect 2, for whether you saved the base or destroyed it."
Like Mass Effect 2, all of your decisions will culminate at the end, and it will reward you for being more thorough. "All the different things that you do, if you do a little side quest, or you go off and do a major plot, these things contribute to the war effort," Hudson explained. "If you just rip straight down the critical path and try to finish the game as soon as you can, and do very little optional or side stuff, then you can finish the game. You can have some kind of ending and victory, but it'll be a lot more brutal and minimal relative to if you do a lot of stuff."
He says that if you take your time through the game, rallying the various factions of the galaxy around the war effort, "you'll get an amazing, very definitive ending."