What about Particle's system's major failure: boredom? No one was ever bored on OldWorld. There was no monotony. If a Lv20 was harrasing you at Lv2, then you turned tail and fled, and made sure to look out. There was also the rivalry between humans and gremlins that kept the server under control. If a Lv20 gremlin started killing Lv2 humans, he would sure as hell start getting slaughtered by the higher human characters, and vice versa.
TRPG is boring without PVP, since other people are the only characters that present a challenge. I'd say about 10% - 20%, at most, of my time on OldWorld was spent leveling. The same goes for most people that played. I'd bet you a large sum of money that figure is reversed on Particles; people spend 80% - 90% of their time whacking away at bots from behind walls.