One of the more exciting prospects of Mass Effect 2 is the way it’ll use your saved game data from Mass Effect 1 to affect the adventure in the sequel. Speaking to PC World (via Shacknews), lead producer Casey Hudson explained in a little more detail exactly how that will work -- and how both small decisions and big decisions you made will matter.
"When you’re playing the first game, everything that you do is setting a variable so that as the story progresses we know that you did a certain thing on a certain planet, and then internal to the game, we can reference those things," Hudson explained. "Your Mass Effect save game contains all of that information. When you import it into Mass Effect 2, now we can continue mining all that information."

The information pertains not just to what your ending was, but to "literally hundreds of things," Hudson said. "Anytime we have a plot or a character or situation in Mass Effect 2, we think about what you did, potentially, in the first game that might affect said plot or character or situation in the second. We can look at each variable and dynamically change what happens in the moment."

By way of explanation, Hudson described one encounter early in the first Mass Effect that can effect how a different plot plays out in Mass Effect 2:

"Conrad Verner was a fan of Commander Shepard’s that you met in the first game, and it’s like you meet this guy in an alley and you can be nice to him or you can be a jerk to him, and at the time you might have been thinking of it as just a trite role-playing convention, good-guy bad-guy, and that’s that.
"Jump forward two years. Now you’re playing Mass Effect 2, and oh my god, who’s this, it’s Conrad Verner! And based on what you’ve done, you realize that while the moment in the first game maybe seemed throwaway, now Conrad’s back and involved in another plot in a game you’re playing two years later...and what you did two years ago is meaningfully affecting what’s happening. That’s a small example."

Hudson also briefly described a few bigger examples, which contain spoilers for those who haven’t yet beaten Mass Effect 1: [SPOILER] "The larger examples are things like...take the way you navigate through the ending of Mass Effect, how you left the galaxy in a certain state with humans, whether they were in control of the Galactic Council or not, things like that," he said. "In Mass Effect 2, when you walk around, you’ll see all the areas affected by your decisions, including large scale stuff like the Citadel." [END SPOILER]

Considering the way these choices can affect choices that affect other choices, and it all splinters out exponentially, it’s really pretty incredible how much consistency BioWare is attempting to build into this trilogy for each individual person playing the games their own way. How well they’ll pull it off, of course, remains to be seen, but we really can’t wait until Mass Effect 2 releases next year to find out.