I overtook a guy 6 times this morning who kept pushing in and jumping red lights in central London.
I've only been commuting for months but both these things are my bugbears.
As has been said by other posters on this thread; whilst not statistically dangerous, jumping red lights gives cyclists a bad name, can annoy car drivers who will take it out on other cyclists, and potentially encourage more cyclists to do it.
Slow cyclists who push their way to the front of the queue at lights really get my goat too. Nearly everybody else has passed them between the last set of lights and these ones, now we're going to have to do it all again.
I'm also not enamored with cyclists riding two abreast on busy roads whilst chatting; cars waiting to turn onto a road and have edged their way onto the cycle lane; seeing cyclists bombing along a pedestrian-only pavement; taxi drivers telling me to f off.
But I still love it