Yes, hate the overuse of inappropriate training equipment, or seeing horses being worked too hard, too young. Quick results are frequently not lasting results, or even good results. Beware the trainer who claims otherwise!
The people that think a horse is worthless because it isn't registered.
It doesn't matter you can't ride papers.
Western Pleasure trainers who train "peanut rollers" you shouldn't change a horses gait, extension and collection are learned through training, not "put the strongest curb bit in you can find and hit the horse's mouth until they slow down then take draw reins and ride the horse with his head attached to his knees"
Saddle Seat people who use soaring and think it is an acceptable training method, same with dressage riders and rollkur, etc
Beginners who are know-it-alls and go around telling people what they are doing wrong, I was a beginner once, but I didn't go around telling everyone what to do.
People who think no horse should be bred unless it is registered and has a perfect show record.
Not saying everybody should breed horses, just that it shouldn't have to be registered and have a perfect show record, I personally don't like most of the breed association show judging standards and show rules, so I don't blame others who also don't agree.