Undigested research annoys me, where it feels like they have just copied something straight out of a book - Jean M. Auel's 'Clan of the Cavebear' books are terrible for this - they these have long lists of Ice Age flora and fauna which she has clearly got from some academic and then copied directly into the text.
My biggest hate, however, is when the author was writing some contemporary book and wasn't quite sure how to end it, and then some big historical event happened, like 911 or the outbreak of WW2, (or even worse, one which merely felt big at the time, like the death of Diana) so they decide to stick it on the end of the book because it felt so momentous at the time, but in retrospect it just looks naff.
There's a David Lodge novella where he does that, and I just finished a Libby Purves one ( I know, but actually they are quite good) which wraps everything up with everyone getting emotional over 911.
Writers are supposed to have detachment FFS!
Oh yeah and I hate hate hate badly written children's books - I always think 'We are going to have to read this hundreds of times, you bastards, how could you do this to us?'
Oh and 'That's not my....' books (if you have toddlers you will know what I mean) where the touchy bit doesn't really fit the word it's meant to. ('That's not my snowman, his arms are so wavy' - corrugated cardboard does NOT represent waviness you moron!)
Ooh didn't realise I was carrying around so much suppressed rage.