There are only 3 times I can think of when I'm puzzled by other riders and drivers not acknowledging me.

1) A road rider that stares right at me and doesn't respond back. If you want to ride by without looking at me, fine. You may be in the throws of intervals or whatever, but staring me down means that when I see you broken down on the side of the road, I'll likely respond following your model.

2) A mtn biker who doesn't say thanks or hello or anything when I've pulled off to the side of the trail to let him/her pass.

3) I'm completely puzzled by neighbors who look neither right nor left when driving through the neighborhood, especially when the neighborhood has narrow streets necessitating pulling over to let someone pass. Dude! When your house is burning down or you have an emergency, you going to look at me then?

Otherwise, to each her own. Yes, I wiggle my fingers, nod my head or smile at many cyclists, and many times I don't.

OK, wait, I need to add #4 - the rider who comes up from behind and passes completely without warning, nearly knocking me off the road because he's incapable of passing more than a frog's hair away. If I can feel your knuckles, you're too close.