It wasn't until I started my orc shaman that I started to understand how MP/5 helps, what good Int is as a stat, the difference between caster stats and healer stats, why some weapons have equivalent damage and healing while others have a lot more healing than damage (or no damage, before the most recent patch) - I've now played two shamans to 70, but that doesn't mean I really understand how the game works for, say, mages or rogues (to name two classes I absolutely cannot play worth beans) much less druids or warlocks.
I'll be honest: I hate those classes. I hate how they play. I hate their mechanics, I hate specific abilities they have. The temptation to hate as an example warlocks, however, is a trap that must be avoided. Sure, we've all run into that warlock who Death Coils in an instance. We've all run into that mage, or that rogue, or that shaman who steals aggro without thinking and who never seems to think of not spamming their highest threat ability when we're trying to pull the mobs off. We've all run into the DPS warrior who insists he can tank and then fails miserably. And with the class system in WoW, it's very tempting to let those experiences color our view of the people playing those classes even if we haven't actually run with them. Heck, to some extent WoW fosters this with the faction setting and PvP elements. There is nothing in this world that I hate more than being polymorphed and having to sit there watching the mage wind up a spell I know will half kill me, my trinket mocking me with its cooldown the whole time.
I had to educate myself to stop being an ass to the people playing the classes I didn't like. They are, after all, still people. Different tastes than mine, yes, but no matter how many bad experiences I may have had with hunters taking aggro by shooting before I'd even touched a mob, with paladins who use DI on me in the middle of a fight, with other warriors who use whirlwind in the middle of our CC, it doesn't justify my acting like an ass to the hunters, paladins and warriors I haven't played with yet, or worse, the ones I have played with and who I know are damn good at their classes. I always tried to keep this in mind, but branching out into new alts definitely helped me with that to a great extent. I still hate playing a druid or a warlock, I dislike their abilities and their spells and the mechanics of those classes. But I've managed to get it through my stupid skull that no one's making me play one, and the people that do enjoy them probably feel the same way about the classes I like.
My next project should be a hunter, I think. If I can just restrain myself from running in to melee and stop trying to take aggro off of my pet, I should be okay.
Have you had to shed any prejudices?