I'm sorry to hear that you have had a bad leveling experience here and there, and it sucks that some people are mean. But I can shed some light on why some of the wow community have become calloused towards new players.
Some of us are willing to help people out when they do not understand a dungeon, i am one of these people. I do not insta-kick when some one says they are new or ask for advice. But one thing that drives me crazy is people that are not willing to learn. Some "leet" gamers are tired of teaching those that do not want to learn.
An example: The other day I was running through heroic BRC. We get to the second fight and i ask before we start, "does every one understand this fight?". Every one says yes so we continue, we assign who stands in front of each laser to stop the adds from transforming. Well some one did not block the laser correctly because the add transformed. so we run back, and mana up, and i ask "does every one know the fight?", they all say yes, so then i ask "hunter do you know how to block the laser beam?", he say no.
The point that I"m going for is that teaching people that do not want to learn is a pain. There are a select few that say I'm new here what do i need to do. They are awesome, but few. Mostly when i try to give advise, or tips i get rude responses, or the best one is when the deny it. "kk this time plz don't stand in the fire", "i didnt!", "i rezed your body from the pile of fire in front of me :(".
It's not fair to new player to get booted and spoken down to, but its not fun to carry people that don't even want to try.