Good question, Mike - but hard to pick just one, too.

I suppose you're talking about published adventures (not written by anybody in the group).  For most of our home-brewed ones, they tend to have way less done up front (much more done on the fly).  Some of us have at least extensive notes, and almost everyone rolls up at least the bad guys/adversaries ahead of time, but a number of our GMs more or less "wing it" (which to me is much different from a published adventure, where the story is written out ahead of time).  Having it articulated beforehand holds the GM to more of a "this will happen unless the players do X" format, whereas if the GM is winging it, they might be more inclined to change and adapt their story on the fly.