Back yard breeders.
IMHO, Just because your horse is pretty, or registered, doesn't mean you should breed them.
I have no personal experience with breeding a horse/raising a foal, but there are so many horses out there, I would not want to breed.
People who insist on bring their legs way out, and kick hard, jamming their spurs into their poor horse's sides. Please, just, NO.
People who insult your riding without telling you how to fix what you are doing wrong. I love criticism, but if you just belittle me for what I'm doing wrong and don't tell me how to improve, then we have a problem. >:(
People who blame everything on the horse. No, you did not fall of that horse because he is evil and isn't trained, you fell of because you didn't tighten your cinch enough. :P
I also, in general, do not like people. But you horse forumers (whatever you'd like to call yourselves) are nice. :)