I tend to go through bouts of not leaving feedback, because I'm lazy and everyone else's feedback is always more interesting than mine.
Yeah, I will admit, laziness is the primary reason I don't leave a lot of feedback, and the other big factor is that I generally have no idea what to say. I try to leave more feedback when I'm reading Numb3rs fic, because it's a smaller fandom, but I'm *bad* at it, I once left a comment basically saying "I want to leave a comment to show that appreciate your sharing this, and encourage you to continue, but I have no idea what to say, so here." She smiled and said I could just leave a &heart; if I wanted. :P

Although, to be honest, I'm not ultra kinky
Says the woman using an icon of Brian tying Ben to the bed. :P

I do read kinky fics, and a lot of AUs and perfect characterizations aren't necessarily on the top of my list of what makes fic good. All are things that a lot of other people turn their nose down at. So it makes me a little self conscious.

Also, if I don't like something, I'm not ashamed to admit it. Certain pairings really squick me, I can't stand mpreg , I'm not fond of most kidfic, etc.
*pulls out special icons, just for you*
But actually, I don't mind mpreg in the right fanoms (QaF was *not* one of those fandoms) and kid fic is great if it's done right. :)