People traveling in the passing lane.
It's not that hard to understand, folks. In the States, the left-most lane (excluding any high-occupancy lanes) is designed for those who are passing travelers in the other lane(s). It's really not all that complicated. Left lane = passing; right lane(s) = traveling.
Here's how passing is supposed to work. You're in the right-hand lane, and there's a car in front of yours, but you're driving faster, so you get closer and closer. You put on your left-turn blinker, check your mirrors, and if there are no obstructions, move smoothly into the passing lane. You stay in the passing lane until you're well ahead of the vehicle you just passed (because it's both rude and dangerous to cut someone off), then you put on your right-turn blinker, check your mirrors, and again, move smoothly back into the traveling lane.
I'm actually a pretty easygoing person. I'm willing to grant that there are exceptions - if you're approaching a left-turn, for example. Even if there's a turn lane, I can understand wanting to slow down and approach it gradually rather than veering across several lanes at the last minute. No problem there. Or if the right-hand lane has a bunch of potholes or otherwise uneven pavement. Totally understandable.
But that is often not the case. Folks are just toodling along in the passing lane for no reason other than carelessness or ignorance, and that grinds my gears.
And don't get me started on people who pair up - one in the traveling lane, one in the passing lane - and mosey on down the road side-by-side for miles.