I've noticed the difference as well. Honestly, I really do think it is the population difference between the two sides. There are jerks on both sides, but there are more Alliance, so thus more jerks (just as a matter of percentage), so they are more noticable. However, I've also noticed that quite a few powergamers are drawn to hunters and paladins (especially paladins), and they tend to be the PvPing, rude people that end up harassing others. You get some of those on Horde, too, but once again percentage wise, Alliance will just be more noticable.
Then again, I've played mostly Horde, so I'm a little biased...
