Just off the top of my head....
People who start horses too young.
People who breed less-than-exceptional-quality horses.
People who try to force their ideas on others.
People who are disrespectful towards other boarders by leaving horses in turnout in a riding area and then disappearing, nowhere to be found when someone wants to actually ride or work their horses, allowing horses turned out freely on the property to run around like wild animals and get other horses who are being ridden/worked/groomed/etc all riled up (free turnout on the property is for horses who will just graze calmly and there are designated turnout areas for horses who will run around), etc.
People who take advantage of horses and don't care if they're just scaring the crap out of them or confusing them.
People who blame the horse for everything and never take on any responsibility for problems.
People who do not respect disciplines other than their own.
People who keep a young or otherwise energetic horse in a stall without turnout.
People who use horses and throw them away.
Horses who have no personality or mind of their own and will blindly follow anybody off a cliff. At least make a person earn that kind of trust first!!
Harsh training equipment.
"Natural" horsemanship, the least natural of all.
Gaited horse soring.
Barbed wire fencing.
Horse pastures with a bunch of random crap thrown in them (tractor parts, old cars, and other random usually rusty things that can kill your horse)
Not having a horse of my own to ride for the first time in years
(had to throw that in there