I try not to begrudge any one class; I tend to be more focus on the person (attitude) behind the character. If the person's giving an honest effort and under-performing DPS, and if i know the class they're playing well enough to help them, I'll try. If I don't, I won't just bash em offhandedly. I save the free-swinging mallet for ninja-pulls and "oh look I'm Mr Comedy over here insulting everyone!"
I have experienced a lot of bias playing my DK, though I admit mostly from Wrath when everyone and his brother wanted to jump on the "oh look new shiny!" bandwagon. It left a bad impression of the class in a lot of people's minds regarding the class, simply because they ran into a person who spent more time trying to face-roll and die than learning the proper mechanics and wearing strength plate. (I remember facepalming like a madman seeing DKs clad in armor stat weighted for a Holy Pally (Int-based Plate) instead of wearing the Strength plate they should have been. Oh and the myths surrounding spell pen in anything other than PVP... I wanted to slash my wrist. I took the time to learn my class, get the rotations/glyphs/stat-based gear and fight knowledge. Sadly, many didn't.. and it leaves a blanket bad-taste for DKs. (Shameful too, given there are more good DKs than most give credit for there being...)