According to BioWare's development lead Casey Hudson (source:
pcgamer; via:
gamesradar),
Mass Effect 3's best ending will require you to actually do sidequests this time. In the previous two, you could speed through the main quest with just one or two choices affecting the ending you get, and you never had to touch the sidequests - of course, doing that meant you missed much of the first two game's content. However, in
Mass Effect 3, speeding through the main quest without ever touching the sidequests will net you an inferior ending. There's no word yet on how "inferior" the other ending(s) will be. Hopefully it will be more than one or two different lines of dialogue. You can click on the PCgamer link above to read the full interview with Casey Hudson.
So finally we'll have some ammo to counter the braggart who claims they saw and did everything but still beat it in 2 hours, making it the worst game ever.
Mass Effect 3 is still set for release sometime early in 2012.