Easy, the role-play.

All other aspects of PS can be out shined by other more glossy, big budget, massively corporate games but, the nurturing of RP is something that those for-profit games can't touch. I know WoW has some decent RP going on and RP servers but, their may goal is to make money. If Blizzard actually enforced all their RP server policies, they'd just end up banning half the people playing... and losing those player's monthly Visa charges.

Since PS isn't in the game for the money, they can actually enforce naming policies, encourage IC interactions, and actually build a game around RPing as much as around PvP, PvE, etc.
So, I guess maybe my answer wasn't so "easy" after all.

I think I'd have to add the fact that the devs are able to make PS however they see best, without worrying about whether it has money-making, super mass market appeal, to what I like best also.
