I live in LA, so:
Traffic.
Slow drivers.
Drivers who need 15 seconds to register that the light is now green.
Drivers who treat STOP signs as red lights. On the flip side of that, drivers in Beverly Hills who have zero regard for STOP signs.
Drivers who cut you off, then drive at like 20 mph in front of you.
When you're on a 2-lane road and the 2 cars in front of you in both lanes are going slow as hell...and you can't pass them or cut in front of them...but there's open road ahead...it's so close you can almost taste the open road but the 2 ass hole cars won't move