Its mostly going to come from players who come from raiding guilds. For example, I can run HM LI in 15-20 minutes with my guild, never wipe etc, but when I pug it; it usually is an hour ordeal spent explaining mechanics and trying to teach someone how to play their class. It's not even elitism, its just that people from higher end guilds/groups end up expecting better and for the most part, pugs aren't close to the same quality as a good raiding guild.
I agree completely being in a top guild. The main other reason is pugs don't have Vent/Mumble/TS3, and if they do people refuse to use them. They don't have to transmit but it helps a lot just to hear. My guild was running with a pug a few days ago and he was new so we asked him to get on Vent. He said "You mean like calm mind?" LOL. That's pugs for you.