I have to start out saying that if a story is good enough, I'll muddle through a LOT of junk. That being said, if the main character's motives change halfway through the book with no apparent motive, that's a big problem.
Also, grammar and spelling issues. If I can find five errors within the first ten pages, someone hasn't done their job and I'm not giving this any more of my time.
I don't mind genres changing midway through, as long as it's not completely jarring.
As for fanfiction, the grammar and spelling kills everything for me, more than anything else. And formatting. I realize that some things are beyond what the author can control (if the website it's hosted on changes everything without them knowing), but I've seen some stories that are literally one long paragraph. No breaks whatsoever, with just back to back quotation marks. Sorry, at that point, I don't care how good the concept is, I'm done.
Another thing along the same vein of page lickers is page folders. People who fold the corner of the page down to mark their spots. Or fold the book open somewhere. Folding open isn't so bad, but the page folders are terrible. I'm slightly OCD and even the corners that get accicdentally folded over during normal wear and tear bother me, I can't fathom why someone would purposely do that. It's mind-boggling.