Comment Eve and DAoC (Score 1) 480
As many have pointed out, Eve is a spiritual descendent of UO, and I think DAoC is as well.
I think the lessons from Eve and DAoC are very simple - you need "timeout" areas; places where people can go to avoid the full-on PvP. Eve also has consequences for being a pirate - you can't enter high-sec space anymore.
Finally, I think Eve has a skill & equipment system that allows new players to enter PvP fairly easy. Unlike most games, it's not simply "bigger=better"; all levels of ships have a role in combat, and quite frankly what you see in low-sec areas (I can't speak to 0.0) is mostly frigates, cruisers, and battle-cruiser classed ships.
So distinct pvp regions and an easy entry for low "level" players is what's necessary.
-Jeff