Practical jokes in general, April Fish Day[1] in particular, and people who think I'm a spoilsport for hating both. If you'd spent most of your childhood being the butt of versions of these jokes that were meant to be cruel rather than funny, you wouldn't like them either.
[1]I have no idea why, but in French, it's actually called that ("Poisson d'Avril"). Thankfully, it's now over for another year.
As for Anonymous, they occasionally serve a socially useful function by attacking individuals or groups that are technically legal but morally reprehensible, but since they seem to choose their targets on the basis of how annoying they find them rather than how nasty they are, the useful function is more a product of coincidence than anything else.