Several...
Trash where it shouldn't be. I hate ballons too, but they were not just dropped there, it got away from someone maybe miles away. When I find an empty beer or pop can laying in the woods, I know some ass hole decided to drop it there.
ATVs. I'm all for private property rights. Riding you quad on your land is your right to do so, but why do some riders (most riders) insist on unmuffled machines? What makes you think you have the right to impose your noise on others?
Competing hunters that just gotta touch everything with bare hands when they walk thru the woods. I try to keep the area around my stand as odor free as possible. When I see someone walking near my stand touching brush, touching rubs and licking branches, standing in scrapes, etc, it drives me nuts. There was a guy that hunted the same property as me several years ago. EVERYTIME I saw him, he was walking thru the woods purposely pulling and breaking branches and stuff out of his way. I watched him do it several different days over the course of a season. One day, I couldn't take it any longer and I said "HEY! Why do you have to constantly touch everything when you walk thru the woods?!!" He had no idea I was in the tree...I startled the crap out of him. I never saw him in the woods again.
Hunters that feel like they need to have a conversation with me while I'm in the stand..."Seeing any?", etc. Or guys that know they are setting up near you but don't seem to care about giving you space.
Bugs buzzing around the eyes and ears. Eventually, one of the little bastards will end up in my eye and if I don't get it out quickly, my eye will swell closed...must have an allergy to them.