A number of people working on commercial MMO design these days got their starts writing code for private servers of UO and other games from that era. The barrier to creating innovative mechanics was pretty low. There were bad parts, but also there were servers that had really great systems and communities.

I remember one private UO shard I played on for a number of years: "Ackadia". Ackadia had two systems that made it unique and wonderful.

If you were not resurrected from your corpse quickly enough it would decay and you would be forced to use one of the UO shrines. Each shrine would only work once. Once you had used all of them, that was it. You'd have to start a new character.

Secondly, monster spawns would spread out from dungeons if not periodically kept in check, eventually spawning in town and defeating the guards. This, combined with restricted travel, tended to condense the players into a smaller number of towns and gave real purpose and consequences to player organization and adventuring.

The result of this was that Ackadia had a tight-knit community, griefing was very rare, and you actually felt like you were accomplishing things other than making numbers go higher: helping and engaging with the community.