Most popular ones I get:

"..."
"WTF"
"that is the coolest thing I've ever seen"
"Why are there 4 of you that look identical?"
"How come all of you have the same gear?"
"HAX. Nobody could possibly coordinate 4 accounts that perfectly."
"I'm so depressed after seeing your kill speed."

I used to reply with the most viscerally caustic thing I could think of. But after the first few weeks I decided I would simply assume people were just misinformed. I wasn't correct, but I was close: most folks in WoW simply lack decent communication skills. Obviously, few people have the wherewithal to conceive of doing this, let along executing it successfully. When talking to people, most actually come around and are nice, or even offer to tank/CC/heal for me if I need it.

In short, the few real pricks will remain that way no matter what. The rest are uninformed, not creative thinkers, poor communicators or some mix of all three.

Now, on a much more interesting note, I have 4-boxed groups of dwarfs, night elves, humans, gnomes and undead. By FAR the most reviled have been the night elves. By FAR the most loved were my 4 gnome mages, Eenie, Meenie, Minie and Mow. An interesting set of social assumptions people make, even in a game where these are _avatars_! /boggle