I understand its a fine line, but the impersonality of the whole situation lends itself to people being less than accommodating for new players. You know you will probably never see this person again, they are on another server and have no recourse with which to combat someones lack of civility.
Heck, I've been booted from a wow gold dungeon group on the last boss just because the group decided they wanted to get their guild's tank in so he could finish his daily before the raid, or because he wanted a chance at a drop from the final boss. I had done nothing wrong, and I cant even send them a tell afterwards to say "what the? why?".
I wish I could offer some solutions, but currently I very much dislike the dungeon finder system. It was much nicer in wrath where heroics filled a very different place in a players world.