Extending this list, because we've barely scratched the surface. :)
by
11: Numerically specifying exactly how many inches a penis is does NOTHING for the person reading your lemon, unless it's in the double digits, in which case, it will make a sane person *cringe*. Similar rules apply for breast sizes.
12: While people *do* talk dirty during sex at times, if you have your characters turn into Snoop Dogg movie extras in a matter of minutes, you're probably doing something very, very wrong.
I think we should compile these a la the SF Mary Sue Test. :)