In my opinion, the worst offender is an almost complete sentence. More specifically, I hate it when a writer writes a sentence that would be complete except for a missing "than", or "or", or a similar word. It often leads to me having to sit there trying to figure out the meaning of the sentence. If a writer would just overuse "like", or fail to capitalize the first letter of their sentences, I could move on.

Robert Jordan is guilty of incomplete sentences in the last two books of his I've read, Wheel of Time #9 and #10. It really slows down the reading. His first several books were nearly perfect as far as spelling and grammar are concerned. Maybe he is becoming senile.