There is bullying on here, definitely. But yes, it's usually involving two or three posters and because it's considered a 'personality clash' or difference of opinions it's largely ignored.
But it does happen, and just because it involves just two people, doesn't make it any less valid.
I've seen posters on here whose posts are constantly criticised/picked up on by the same other poster over and over. On it's own, a single critical/scathing post on a thread, I suppose it does seem innocuous, but when it happens time and time again on threads all over the place, that's when it becomes bullying, of sorts.
It's difficult to quantify though - different people have different ideas as to what constitutes bullying. I happen to think that one poster constantly hounding another with contrary opinions or snidey comments relating to other threads/past history, IS bullying. But some people see it as acceptible. Just that they have two wildly different opinions and both like to state them.
I think if you FEEL bullied, you usually are being.
I think MNHQ's stance on self-policing is a relatively good one. It is much more difficult to uphold though, the more traffic that comes through here. There does lie the issue though, as I saw just the other day, that if a post IS deleted having been reported, then the author of the deleted post then rounds further on the person to whom their personal attack was directed (irregardless of whether the attacked poster reported it or no).
I suppose you can't mitigate for individuals reactions though.