I say this unironically, but a lot of bugs. The bugs are what made UO fun, and the team often treated the bugs as features because the community demanded it. The most famous example (I think) is the "true black" dye. Another bug I ran into was when a certain shade of brown hair got turned "true white," I was able to petition a game master to let me keep my true white hair after the bug was fixed because I made it part of my "persona" at the time.
Also there was a whole niche industry of collecting non-droppable items which spawned in the game world but were not fixed on the map (we think they were added post map creation), so they could be "pick pocketed" off the surfaces they were on and taken back to your home every wipe. There was a huge rush after servers came back on after a wipe for folks to go find the most rare items to stock up their towers and keeps.
UO came out when I was in high school. I would time my morning routines around the server reboot to grab those special items, some of which didn't respawn during routine reboots.
The bugs were part of the game culture. The first time that you learned that items in the bottom right corner of your first house -- because they could be stolen through a bug even if your house was locked -- was something everyone jointly went through.
UO also had maybe the closest thing to a true player economy than any game. There was a legitimate path to making money (and having fun) to just mining ore and selling ingots. You would sell your iron bars in an unattended vendor to other players at your own price. Those bars would get bought by a blacksmith player to produce armor that they sold to other players... who would buy it to go adventuring in the dungeons.
How does the economy compare with the structures in EVE Online?
It was smaller scale and simpler, easy to understand without spreadsheets. Even a middle school student could understand their slice of the economy.