The first thing I look for (unsurprisingly) is the summary. I don't read a huge number of fandoms, and those I do read tend to lend themselves to serious amounts of plot mixed in with the erotica. I like plot as opposed to PWP.
I both read and write slash and het. I'm not overly fond of femmeslash, but I will read it if it's well done, and not just an excuse to parade girls in strap-ons or futanari. And at the first sign of one female calling another her "ass-bitch" or "anal slut," i'm gone. Sorry, but I don't care for women who emulate the worse of male behavior.
I don't like rape when it's presented as enjoyable, or an excuse for a character to decide to fall for their rapist. Really? I'd be much more likely to want to see the son of a bitch dead in a dozen really creative and hideously painful ways. I can accept rape as a plot device, I prefer it to be presented as the bad thing it is, and I certainly don't squeal with happiness when I encounter gang rape.
Same thing with incest, for the most part, although I will happily make an exception for Elladan and Elrohir because that is just too delicious a bit of twincest to ignore.
So what DO I like? I like characters that have flaws, characters that are complex and interesting, and plot. Lots of plot. I like women who aren't always beautiful, who don't have every man lapping at their feet, and who can take care of themselves for the most part. I like men who can look a woman in the eye, or who can have a relationship with another man without turning into a queen who cries during sex. (There is NO crying during sex.)
That's a bit, anyway, of what I like.