People mistreating workers. Retail workers, waitrons, clean-up folks, people that they don’t see as people or who they think are “below” them. If you’re interacting with someone who is at their job, they are generally obliged to take your crap – they can’t run away, and they need a job – but it doesn’t mean you are obliged to be a jerk. A service worker is a person just like you.
Other things piss me off too, like all the stuff @Bluefreedom mentioned, but that’s always there. The service worker thing irks me on a more immediate level.