Most people in WoW don't want to allow you to do anything until you've already done it. Thus, unless you're very lucky, you'll never be able to do most things. It's why such a small amount of people raid, do RBGs, and why arena isn't as used as it should be as well.
In other words, most people in this game are massive pricks.
On one hand, I see people going "WANT 2k RBG EXP" while being 1300 arena players, and haven't even done a single RBG themselves, but just want to get carried. Same shit goes for PVE, and people making PUGS for raids.
On the other hand, if they're looking for 1800 players, and their MMR is 1800 there is no point in bringing a 1500 player with them that can't compete on the same level.
Though I avoid RBG's generally. In Cata I was playing 2.3k on my mage(3's), and this guy was asking for a 1600 RBG, and I he wouldn't let me in, because he thought I wasn't good enough or some shit like that.
Yeah it's beyond ridiculous at the moment. I frequently have to log my second account and link some pvp achievs to get invite and even then i sometimes don't get in because it's not on the character i'm joining with. And i'm all like "dude you are an 1800mmr player" It's a complete joke.
I see terrible play every day at 1800ish rbg. Can't believe these guys have the NERVE to be elitist.