Ketchup and mustard. Don't care for either, extremely so in the case of ketchup. Mostly just want to make ordering hamburgers easier. Also don't like mayo on burgers/sandwiches, so I guess throw that in there, too.
I hear ya on this one. I eat burgers/sands with only meat and cheese. Gives about 82 percent chance of the place getting it wrong.
I find ordering a plain cheeseburger is usually fine, but I like lettuce and tomato and stuff, and if you order "no ketchup/mustard/mayo(looking at you, Wendy's)" then they eff it up more often than not.
I learned why when I was working at Arby's, though. There's basically one button press in the system for plain, but if you specify just certain ingredients it takes longer to input, and the burger/sandwich was already up on the screen for long enough to start making it. I assume most other fast food places are similar.