Handshakes in general. They generate this nagging reminder in the back of my head to watch my hands at the closest opportunity, given that most people I shake hands with are men and too many men walk out of restrooms without washing their hands.
There's also the mystery handshakers, where you don't know if it's going to be a classic or cool handshake so end up half-raising your arm while you try to figure out which one it's going to be, if any at all.
And the grips. I find myself trying to judge whether or not the person I'm shaking hands with is normal...or if they'll try to engage in a bone-crushing contest. As a result, my own handshake's pressure is neutral, but stiff and ready to counter-crush.
The duration...that feel when you go to withdraw from a handshake but the other guy just isn't fucking finished yet.
