Misplaced apostrophes are bad, but I'm annoyed by people who use *no punctuation at all*. They usually don't bother to capitalize words either, so their posts are just a jumble of run-on phrases, like a speed freak's stream of consciousness.

Dood: did you ever hear of the 'semicolon'? Seriously, it'll change your life...
Some people don't see the big deal about bad grammar, as long as the ideas get conveyed properly, but I feel good grammar is a sign of courtesy, like sending a letter on nice paper, instead of ratty foolscap. Plus, it's hard to be sure ideas *are* being communicated accurately if people aren't all using the same linguistic conventions.