Or rather, the lack thereof. UO servers were limited in population to a relatively small number (at least relative to today's MMORPGs). When playing UO, the map was small enough and the playerbase was small enough that you could be reasonably certain of who you'd run into at Brit GY or Wind or wherever. They eventually changed the reputation system a bit, and while getting rid of dreadlords didn't kill the game, obviously, I think it promoted the rise of ganking, which is a shame.
Another great thing about UO were the server-down fights. Yes, they might be annoying to deal with now, but being able to fight with wild abandon, with no fear of actually losing stuff, led to some incredible moments - and no doubt, was helpful for both game balance from a dev perspective, and new players learning how combat worked.
As far as "catch-up" time to get started with a new character - it was insanely low. Sure, your new character was likely to get his ass handed to him, but it was also possible to start a brand new character and kill a fair proportion of players who were of much higher experience, simply because they sucked at fighting. Yes, UO was a skill-based system, but more than that, combat required *skill* - and I'm not talking about learning the timing necessary beat repetitive and predictable WoW bosses by grinding raids until your clan can run it in their sleep.
And there were relatively few moments when one build of character was "the best" in the same way that you see guys relentlessly optimizing their talents and gear and all that other bullshit. Okay, maybe viking swords were the shit, or you'd want to make a dex-monkey, but you could also do something wild like a bard, and the game could still be fun.
And for all that PVP was a risk, there were plenty of players who had master craftsmen, who mostly chatted with their friends by the bank and never really saw that much combat. Which was fine, because there was also a legitimate (perhap's online-gamedom's only) player-run economy. It's also the only MMORPG I've played where players actually look different from each other, with no real need for all-names or tags, and...
God, now you've got me missing UO all over again.