The answer is a combination of everything you guys have said. It's anonymity, no repercussions, and immaturity all combined in a perfect storm of asshole behavior. I had responded to a guy on the PTR this morning who had said "This is so stupid that Dalaran doesn't have any portals". So I sent him a tell, and we went back and forth for a bit (in a good way). My opinion was that Dalaran was going to be a ghost town, since most people are going to be in one zone at a time, in either Eastern Kingdoms or Kalimdor, and that perhaps he should wait to see what Cataclysm brought before being so judgmental without actually experiencing the content, and therefore to see how much of an impact (if any) losing the portals in Dalaran are. I think I got through to him, but that's rare. Usually you hear them spew any hateful shit they can think of, which is when I go ahead and report them. I also stand up for guys I see being abused in trade chat, and the like, including sending people tells asking them if that was really necessary, and explaining (as was mentioned here) that everyone was new at some point, and we should encourage, not discourage new players. The response is mostly negative, but that won't deter me from sticking up for the "little guy". Someone has to.
I also have a RL friend at work who clearly doesn't "get" the warrior, based on the fact that he is spec'd arms, yet has tanking, PVP, DPS, and Frost resist gear on in the same set. I do what I can to help him, but I'm limited because he plays on another server. I think if we *all* took some responsibility, helped people as much as possible, and didn't join in when the hateful assholes descended upon the helpless newbies, we'd give these pricks something to be mindful of.
My 2c.