Hmm... I like my butt cheeks grabbed and then pulled apart when I'm riding a guy. It makes my ass feel great. It's easy for me to say what I want, but sometimes it's harder for me to say what I don't want (because talking about what you want is sexy, talking about what you don't want is a downer).
The advice that I would give for people trying to find out their partners' turn ons would be to set up an atmosphere where it is safe to talk about these things. There are a couple things you can do for this.
1. watch porn together and ask "would you want to do that?" for everything that happens. This lets you find out BOTH the good and the bad, and inevitably, you get to act out your own porno in the end
2. Don't insult other people's kinks at any point. This means that if you are hearing second hand stories about "friends of friends", or even watching TV, think about what you're saying. You don't have to say that everything is hot, but if someone is talking about a sexual practice, say rimming, and you go "eww! why would anyone ever want to do that? that's so nasty!" suddenly you may have alienated your partner without realizing it. If you say "I know a lot of people find it pleasurable, but I don't know if I would be comfortable doing that." then at least your partner doesn't feel like a freak, or he/she can open up a dialogue and say "if the person showers first, it doesn't smell bad, and you can use scented lube, etc" and suddenly you are opening up lines of communication rather than shutting them down.
3. Say things that YOU like. Communication is a two-way street, and when you start opening up about your kinks/desires/wants, it helps the other person reciprocate. Besides, maybe your wants are also your partner's wishes too.
And if you are still reading, I am impressed. Just to add a final note to this lengthy diatribe - sometimes it's not what you do, it's how you do it. Keep this is mind when you are asking questions, because if the girl says "grab my ass", does that mean in a soft, gentle, loving, massage like way, or does that mean in a rough, primal, caveman want girl to ride him harder sort of way? There are subtle nuances that you should keep in mind, and glean from the conversation.more