StackOverflow is awesome and nothing touches it for quality as far as I'm concerned. But the one thing that annoys me most is the seemingly arbitrary way that posts get deleted by Moderators after they have been closed (but still visible) for a significant period of time. Often they are extremely popular posts with 100k+ views that just disappear in a puff of smoke.
FYI: An SO user has recently setup an unofficial archive of deleted SO questions, ranked in order of votes ... http://meta.stackoverflow.com/questions/124850/unofficial-st...