The thing that gets my hackles up:
kewl d00dz \/\/h0 TyP3 EvErYtH1Ng LiK3 Th15
translation: "Cool dudes who type everything like this", aka "d00dspeak". It's more maddening than people who use "u" for "you", "r" for "are", and "plz" for "please" - which also drives me over the edge.
IceWolf