There aren't many things that really ruin RP for me. In a sense, that is. Generally I'm quite optimistic about it all and if I want to enjoy some RP-ing, then neither questionable characters nor rude and arrogant players will stop me from entertaining myself and others.
Of course there's plenty of stuff I dislike: people mixing up IC into OOC and unleashing a horde of drama llama's because of that, people going out of their way to -attempt- to ruin RP being the most notable ones. Or ooc politics and whole conspiracy weaving, ouch my head.
I'm not likely to voluntarily approach 16 year old characters that are fully fledged evil overlords or what not, or people who clearly just discovered the f-word and use it about every 3rd sentence to appear badass. Somehow I dislike this even more when used by non-humans.
Also, people who yell 'racist!' as soon as you tell them that you distrust Gilneans, hate orcs, or something like that. That includes Alliance characters who somehow think of the Horde as peaceful people who are only misunderstood. Of course as players we know that they (or some of them) actually are, but it always irks me a bit when characters do it and cannot provide a solid reason why.
Another thing that I find a bit meh, is that I often have seen roleplayers talk with disgust about raiders as all being 'lollers' (ugh, that word) and the weird idea that playing actual game content is below 'true' roleplayers. I have spoken to quite some raiders who really wanted to roleplay but were kind of anxious to try, because of all the snide comments they received in the past. And thus, they were on a roleplay server but did not roleplay, and so the vicious circle went on.
Though in the end, none of these things really ruin roleplaying for me, because I don't let them. And worry not, I'm only fussy and picky towards my own characters' lore and history :-)
P.s. I noticed that kaldorei / kal'dorei thing above. Aren't both spellings correct? Night elf lore is one of my favourites (after human paladins of course!) and I recall that I've seen them both used ingame, but that could be me. Since if the real spelling is kaldorei, then I'm definately guilty of spelling it wrong for a loooong time. Whoops!