I've got a new one for the list! People that don't use their lights in poor lighting, be it fog, heavy cloud, rain or low glaring sun that puts things into sillohette.
Tip, if the light quality/ angle is bad and you're finding it hard to see the vehicles ahead, the drivers around you will also find it hard to see you! Especially in one of the dull monotone colours that have been so endemic for the last decade (yes this includes my own car!)
DLRs are of limited use for this problem, while they are useful for the front, sometimes, like when driving into low sunshine, you could do with the extra illumination on the rear.
I think most of my others have been covered... indicators, tailgaters (especially the ones that inch past after you've pulled in for them appropriately, that then sandwich you behind a slow vehicle because you'd anticipated that they were going to pass at 80 mph not 70.00001 mph from the BMW/ Audi/ Merc badge that was 3 inches off your rear bumper and you thought there would be enough time to continue at 70mph to pull back to lane 2 before catching up with the Micra doing 57 mph.
Ooh, another... people not using slip roads to accelerate to the speed of the traffic they are joining. On a related theme, a few miles away there is a dual carriageway that is a major route across hundreds of miles across the UK. It also forms part of the local by-pass. I hereby nominate the muppets that join this major route at the roundabout and gradually accelerate to 45 mph (NSL) Now there's a bit of a climb over some bridges, nothing spectacular, but it does involve dusting off that right hand pedal and getting a bit of value out of 3rd and 4th gear in order to get to an appropriate, safe speed for a NSL. But no, stuff that, they're just having a nice gentle pootle along the local by-pass for a few miles. 