Many things. Many, many things.
People who don’t use their turn signals (as I’ve said somewhere else, we’re not mind-readers, and that switch on the side of your steering column isn’t there for ornamental decoration).
People that zig-zag across more than two lanes (and then back again), at high speeds, without using their turn signals.
People on the freeway who use the shoulder or a lane that is ending/merging to try to speed ahead of everyone else, and then cut in. Hello: YOU are part of the reason why that particular lane isn’t moving at all, you selfish prick.
People who intentionally run red lights.
People who are on the phone/texting.
Like many of you, I am not a fan of tailgaters, especially the more aggressive ones.
People who make illegal u-turns in the unsafest of places (curvy streets with blind spots; just beyond major intersections, across all lanes, during rush hour).
People who speed down the opposite left-turn lane, crossing the double yellow lines, trying to be “first” in the left-turn lane up ahead. These folks are just begging for a head-on accident, or a very expensive ticket.
People who are doing anything but driving (I have seen people eating with both hands while steering with their knees, women applying makeup/mascara, men shaving, people reading books (!!), people flossing, and parents looking back behind them while driving at full speed, lecturing their kids or, in one case, helping with homework (no, I’m not kidding).