This evening I finished reading A Song of Stone, by Iain Banks. I've enjoyed some of his other books, but not this one - I didn't like the writing style, though as it was narrated in the first person it was probably intended to indicate that the narrator is insufferably pretentious, but if that was the case, it did its job too well for my blood. That said, it helped me pinpoint something that hasn't sat right with me in a lot of novels.
My Leaving Cert English teacher said that sex is easy to write badly because it's a repetitive act. I think it's fairest to compare it to eating - they are both carnal acts enjoyed by a majority of people. When people write about eating, they talk about the taste of the food, how it made the people eating feel, the sensations involved in eating. By and large, the descriptions are not of the food being cut, speared on cutlery, chewed until a bolus is formed, then swallowed. When most novelists write sex, however, the focus is on the actions taking place. Different strokes (wahey), and one can argue that sex scenes don't strictly have to be titillating, but I feel if you can write a sex or food scene that is simply boring to people who aren't asexual/anti-food or anhedonic, it's possible that your approach needs tweaking.
Nor do I consider descriptions of appearances/actions full of awkward, cringe-inducing metaphors to be addressing the correct issue, but that could just be me. From A Song of Stone (Chapter 3, page 37 in my version):
Exposed to the night and to my mouth, your breasts were moon-pale, and down-smooth, their aureoles and nipples as dark as bruises, raised, thick and hard like a little finger's topmost joint;....
Call me old-fashioned, but I feel that if your description of breasts causes one person to think about severed fingertips instead of about boobs, that is one person too many.
... Really, the mental image returned to me every time that character was topless.