Raph Koster, aka Designer Dragon, orginal designer of Ultima Online post a long essay on lessons learned from UO. Here's the best part.
In an odd sort of summary... Being safe from evil is, in my mind, an uneven tradeoff for the fact that you don't get to be heroes anymore, in that you can just opt out of fighting evil. It may be nobody wants to be heroes except when it doesn't count, when it isn't challenging, that people would rather fight "pretend evil" than the real thing, but I don't personally believe that. I still think people are better than that. I know this is an odd and probably controversial (perhaps even stupid) position to take, but it's how I feel. I think that the greatest value of interactive entertainment is when it engages you for real, and teaches you things for real.
He's right. And it is exactly why I don't like Trammel or games with PvP switches. I want to be a hero when it really matters.