steve:
Well, there's the interaction things, the eye contact, interesting conversation, little BS, no complaining about BFs, etc. That drew my initial interest, and convinced me to spend more time. All of that is easily feigned, but more time revealed either a very skilled actress (still a possibility, of course), or someone who is genuinely intersting and relatively free of the usual SS. There's plenty of girls who can fake those things, though, so that alone doesn't account for it. She also has a tendancy to touch herself in all sorts of interesting ways, and doesn't appear to be self concious about it.
One of my biggest turn ons during playtime Is a woman's reactions to arousal, and especially, orgasms. Most of the girls I've met are relatively adept at faking such things as a soft moan, or breathing hard or even moving their bodies, simulating arousal, and making them seem real. It's not so easy to fake things like the hardening of a nipple when I circle my tongue around it, the way the inner lips open up when sufficiently aroused, or the perfectly timed twitch as my finger finds that spot inside. What's even harder to fake is the flush of blood to the chest, and the speed and intensity of the vaginal contractions during orgasm itself.
Not only does she exhibit all the signs of arousal, but she isn't all all reluctant to be aroused. The fact that I can have that effect on her is intensely satisfying to me personally, and the memory of our last encounter still makes my stomach tight and sends a tingle through me. She's so relaxed and real about the whole thing.
With most girls there's a veneer, though sometimes an honest, if not very deep, one, of enjoyment. The apparent sincerity (and I still have to allow for the possibility that I'm being snowed, however unlikely I think that might be) of this girl is what makes me hot to trot, as they say.