Any horse that doesn't walk at my shoulder when I'm leading drives me crazy - whether they're pulling ahead or dragging behind. Also, difficult to catch horses, horses that bolt through the gate when you open it, and horses that don't have a clue how to steer bother me.
However I have to remind myself constantly that it's not the horse's fault - it's the lack of training they've had. So instead of thinking "Boy I'd love to beat the crap out of this horse!" when I'm frustrated, I think, "Boy I'd love to beat the crap out of whoever tried to train this horse!" :P It's a good way to refocus negative energy. :-D