People who, for whatever dumbass reason, stop 2, or even 3 or 4 car lengths behind the vehicle in front of them when stopped at a light. There's just no logical reason that I can think of to do that. It's worse if they do it in a left turn lane. The left turn light doesn't last forever, and you staying that far back just means one or more folks won't make that light. Very inconsiderate IMO.
I mentioned this earlier in the thread but, IMO, it bears repeating: I despise people who interupt someone else speaking. Nothing says rudeness like "I'm not even paying attention to what YOU are saying and what I have to say is of much more interest/importance than what you're saying". Just royally pisses me off. Personally, I won't let them off the hook, I just interupt THEM and begin my conversation anew.