What grinds my gears:
Bad barn owners- I was at a barn that would put dirty wood shavings in the arena and call it proper footing. Yep, and there were piles of poop and flies everywhere. A compost pile is fine, but really? An arena where people BARREL RACE? Not to mention that arena was a full mile away from the pasture and they would feed hay with foxtail and sometimes straw and charge $400 per month . Got out of there pretty fast. Not to mention the horses kicking down rotting fences and getting scraped by loose wire and the owner not responding to email or phone and not ever being at the ranch at all. Eughh. Worst place ever.
People who have horses but don't care for them- If you don't have the time, money, abilities, etc to check on your horse or at least ask someone to, you really should not be owning horses.
People who never learned the "sharing is caring" rule- My old drill team was in one arena at night, some woman came is and started walking her horse around. That would have been fine of course, the horses were well behaved and we were using the covered arena with the best footing while it was raining. Then the coach walked over to politely tell the woman we were doing drill team and would be using the inside of the arena (telling her we would be giving her and her horse a wide berth on the rail- no easy feat in a small arena) and she FLIPPED. Started ranting about how she owned "sooo many horses" at the barn and she should no be confined to the rail. Well, there is another covered arena and you came into an arena with seven other riders already performing a drill- what did you expect? And all she did was walk her horse around.