My pet hates:
1) Tailgaiting. It's just TOTALLY un-necessary and achieves nothing positive. I am with Jonnycool as to the reason this happens. Selfish, impatient, thoughtless people who are thinking about nobody other than me, me, me, I don't care about the speed limit, I want to break it and you're in my way, so I'm going to sit on your @ss......
2) Lane discipline on dual carriageways and motorways. If you are not overtaking something, not gaining on something quickly with a view to overtaking it, then MOVE OVER! Equally, when you're driving at 70 in lane one, then a numpty pulls up alongside you and matches your speed / fractionally overtakes you, but takes forever. This means you are now gaining on a lorry, you want to pull out to lane 2 to pass it - and you can't, because this numpty is now sat there, doing 1mph more than you... you can't really easy off, because another numpty is up your chuff, and chances are he's going to try and pull out to lane 2 first...
3) Texting and driving. I overtook a car on the M5 up by Brum a few weeks ago. I was in lane 3, overtaking. The young lady driving the car in lane 2 was totally engrossed in tapping a text message into her iphone, ocasionally glancing at the road ahead. Asking to maim/kill someone, and as Jonny rightly says - it's because of the "me me me" attitude, I haven't even taken time to think of the consequences of killing someone by texting and driving, cos all I care about is sending my text message so I'm a popular blah.....
4) Excess use of speed combined with bullying - mainly on motorways. Don't get me wrong, I'm actually not against speed on it's own, and wouldn't give a hoot about someone driving over the NSL within reason... apart from when there are aggrivating factors... for example, Mr Bloggs is doing an indicated speed of 80 in lane 3, nicely passing the van in lane 2, before he's ready to move over... when Mr Small Man Syndrome in his 5 series comes caning it up behind him at 130mph, looking like he's going to literally push him out of the way, and sitting right on his @ss, often with main beam flashing....
They are my main ones!