For what it's worth, I don't mean "Oh, I bet the hero and the bad guy fight at the end" speculation which is kind of... obvious. xD I mean things like figuring out The Usual Suspects, Se7en, anything like that - figuring out the ending before it happens and you lose any sense of surprise. When a movie is designed around a dramatic twist and you've been told the twist already, it ruins it.

EDIT: On a sidenote, just because everyone should have seen Avatar by now doesn't mean everyone has, so I've marked the relevant spoilers (even if some were obvious enough to me xD). It's not even finished showing at the cinema yet!