The RenFair kiddies absolutely kill me. One kid at our games had a Renaissance Hat, the ubiquitous poofy shirt, tartan plastic tablecloth kilt, sword (non-Scottish of course) and moccasins. The worst thing was he was running around talking to everyone in an extremely poor, fake Elizabethan-English, and generally behaving like the drama student he probably was If you want to look like an idiot, ok that gives us something to talk about, but don't run up to everyone you see and blather at them.
Absolutely intolerable are the NeoNazi types. A few came to the Gatlinburg Highland Games one year, and were basically told by everyone where they could go (hint, it wasn't the beer tent), and we haven't seen anymore of that trash.
Last is the music. Albannach/Clann an Drumma, Flogging Molly posers and the like; I don't like punk music, or the same bagpipe and drum song given different names and played 10 times in a row. Less of this crap and more music like Silly Wizard, greatest Scottish music EVER.