Stream-of-consciousness, dialects, unnecessary invented words, and anything else that forces you to stop and puzzle out just WTF the author is trying to say.

Blatant hiding information from the reader: especially changing the scene right before an obviously important conversation, or deleting important dialog from the scene. I get really annoyed if the missing information isn't revealed (or deducible) by the end of the book. Does anyone think Robert Jordan still remembers all those schemes that were started in the prologue of The Great Hunt?

Arranging scenes out of chronological order, especially if done specifically to fool the reader.