Not plausible is my biggest pet peeve. If you are a reader of fiction, you're expected to have some suspension of disbelief. But I find some people read fiction just to try to find things that "couldn't really happen". I find that these people are often hardcore gamers. I have nothing against gamers (I like RPG's myself) but they seem to get into a "system" mode and when they find things that don't go explicitly along with that system, they get upset. I'm not saying that speculative fiction books should be outrageously unrealistic; there's clearly a certain balance that has to be struck. I just think in the act of reading fantasy or sci-fi you should let the writer weave an illusion for you, not go looking for errors.