I like certain gen, depending on the characters. Snape and Draco, Londo and G'Kar, Sanzo and Hakkai for instance, where the interaction and history between them lie thick and rich. I find some of the better fics that explore friendship eventually decend into slash, which sometimes actually dissapoints me, 'cause it either didn't need to be or it disrupted the relationship balance there originally. Othertimes it's done wonderfully and I'm just in piles of wibbling happiness.

Other pairings by favorite authors are hit or miss, depending on the pairing itself though there's a higher chance that I'll like it.