Sombreros: Perhaps not as racially sensitive as the American headdress, but equally as annoying when you can't see the stage through some burk's large headwear.
Spending too much time on people's shoulders: By all means, get up there, have a sing-along, get a good look at the band, but don't take the mick, you're blocking everyone's view and likely to become a target for those aforementioned cups of piss.
The shouting of 'Alan': This should be outlawed and banished to the year 2004.
The throwing of cups of piss: Seriously. Evil.
Camping out in the main arena: OK, we all need to rest from time to time. This gripe is mainly for those who reserve the right to lay campchairs and picnic blankets in tented stages or in the busiest areas of main stages and act like they've got the God-given right to vast areas of land around them - blocking access and making it harder for everyone else to enjoy what's going on. Why not just lay a picket fence or some barbed wire? At least don't tut and moan when people attempt to get past you.
Filming on phones in general: Kate Bush had the right idea, banning phone footage actually forced the audience to enjoy the show through their own eyes rather than 3 inches of screen. Also, what's the point? If you're at Glastonbury or Reading, chances are that your footage won't be as good as the BBC's.
Shooting stuff on an iPad/tablet: Good to see you're dumb enough to bring a tablet to a festival. Is there anything larger or more cumbersome you can hold in the air to block our view?
Air Horns: A calling card for the most obnoxious of cretins at festivals.
Massive VIP areas near the stage: You've waited months to catch a glimpse of your favourite band, only you can't really, because you have to stand an extra 100ft further back to allow for a special area sparsely packed with the cast of Hollyoaks. They're better than you, you see - they deserve a better view.
Flares: Hey! Look at that bright red distraction! Man, now they have our attention. I wonder if they want to be mates?
Flags: Sure, it looks pretty cool on telly or from the stage when there's a sea of flags adorned by increasingly keh-razy logos, slogans and in-jokes, but they're a total pain in the arse for anyone stood behind you trying to see the band. What are you trying to prove?