Lack of chemistry.
I hate that they just throw together two people who have never met, and expect them to be intimate with one another, and for that emotional content to translate on camera.
When there is an obvious rapport between the people being filmed, the performances turned in are twenty times better than where there is none.
I also hate it when people don't know how to talk sexy or talk dirty in a scene. I LOVE dirty talk, when it's done right. HATE it when its not. Learn how to communicate during sex, please.
I hate it when it is obvious that someone isn't into what is going on in the scene. If you are not into it, don't do it, because if you are doing it half-heartedly, you will turn in a shitty performance. I want to see gusto and enthusiasm, not wishy-washiness.