My one pet peeve (ha! like there's one) is sloppy writing. By that I don't mean the random misplaced comma or dangling modifier, which, hey, it happens, even with the best of editors. I'm talking about events in a story that could easily be taken care of, had the writer thought for a second about what needed to happen.
My most recent example comes not from a book, but from a television show: Battlestar Galactica. On a recent episode (and the following might be spoilers - consider yourself warned), a member of the crew keeps looking up at the hills. Eventually, he walks up the hills and discovers a magic temple that's, like, the answer to all things, or something (I'm a casual watcher at best, can you tell?) So, my complaint is this? Why the hell does this guy keep looking up at the hills? What's up with that? It's just plain sloppy. The situation could have been handled in about, oh, a HUNDRED different ways. Maybe something from the magic temple glinted in the sunlight, perhaps? Maybe there are turbines in the temple, and the character hears them whirring in the background, something that he would be particularly attuned to as a result of working with machinery? Or perhaps, because the crew is on the planet to gather food, the crew guy wanders up the hill to check out whether there's anything edible up there? Seriously, there are so many different ways that the writers could give the character a reason to walk up the hill and, instead, they just ... don't. Sloppy.
(Yes, yes, it's television, so there's a possibility that the writers do have an explanation for the crew member being "drawn" up the hill and we're just not privy to it yet - Joss Whedon taught me that one in season five of Buffy - but the fact remains that this is just one apparent example of a common problem.)