When it comes to driving, I don't mind as much what happens out in front of me, as long as I can react to it. There are dumbshits everywhere, of course, and as long as I can pick my way around them, all's okay.
But what really gets me are the ones behind you that you can't control or avoid - the tailgaters, particularly those at night with bright headlights (whether they have their brights on, or have those high intensity halogen lights, or just have their headlights aimed too high - it's all the same general category of a-hole in my book). I'm talking those guys who are just a car length behind you when you're already going 5-10 mph over the speed limit; totally unsafe.
Of course, in the egregious cases, I usually just slow way down until they finally pass (if I'm on a multi-lane interstate), but it's still molto annoying.