The one thing that really irritates me is when someone pulls out right in front of me, only to go 5 or 10 mph under the limit, and there are absolutely NO cars behind me (they could have waited three seconds for me to pass and gone as slow as they wanted).
This happens to me quite often on my way to work. I typically beat the morning rush, so I'll be the only car on a particular road (one lane each way). There is a traffic light, and any car coming up to that light will gun it around that corner to get in front of me (to the point that I have to hit the brakes to avoid rear-ending them) and they end up going 35mph (it's a 45mph zone).
It's incredibly irritating and I've started putting the beams on until they GTFO of my way or speed up.
Also, people who sit 2 car lengths behind sensor lights. People, they've been around for decades, and your car needs to be over the sensor to trip the light. Let's move up. Thanks