If starting from scratch, what would an MMO need to replicate that?

I've experienced it first hand, but I can't grasp why it worked well like it did.

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.

You’d need to start with the premise that combat shouldn’t necessarily be the focus of the game. Work on making other aspects (farming, hunting, taming animals, etc) to be equally compelling mechanics.

I still think 90% of what made UO unique is the fact there was no Google or central repository of expert knowledge. Yes, UOStratics existed, but it wasn't perfect. A lot of the fun was in the fact basically nobody knew the BEST way to play, and therefore everyone was just doing whatever they thought was fun.

UO didn't have a global concept of a level. You had a maximum number of points per character, which you allocated to skills by doing the corresponding activity. This is how you can skill cap your character without killing monsters or players.

The big problem that UO ran into was that it turned out the people who liked what UO was is a pretty niche audience. In a lot of ways Everquest was a direct rejection of the features that folks like me think of as the golden years.

But to answer your question, there are three different clusters but contradictory sets of answers. And this was the problem.

1) It was a sandbox game developed with a focus on recreate a living world. A real ecology, real economy, skill based character system instead of classes where your skills tracked what you actually did, a focus on all sorts of roles - part of the original pitch was players could be the town blacksmith or whatever. I knew someone who spent several months playing an interior decorator for instance. Some people, such as myself, were attracted to this.

2) The same freedoms from #1 attracted PvP style gamers, especially from the then nascent FPS style games. Griefing, rampant slaughter, that sort of thing.

3) It also attracted PvE players who weren't at all interested in a realistic world and demanded the sort of conveniences we see in modern MMOs: mobs pinned to locations, predictable drops, predictable quest lines, instancing, optional PVP, etc.

You'll note that most of the people you see reminiscing online are from groups #1 and #2. Group #3 by and large hated the game and left as soon as they could. And your typical group #1 player eventually got annoyed at group #2 and just left altogether.

It's a hard problem to recreate UO because of this tension. Without allowing group #2 to exist you don't have the same environment. But by allowing group #2 to exist, they'll eventually take over and chase away everyone else.

At the end of the day, UO was a game that was simply a moment in time that can never be recreated. Too much of what made it great was due to the fact that it was a new thing.

Is scarcity part of it? Making sure there are some jobs people have to do that don't involve combat but still drive the in-game economy?

Well, it was kind of the only game in town. Sure, you had Dark Sun Online and Meridian 59, but in 96/97 you were playing UO if you liked the idea of online worlds.

I think the barrier to entry is the equivalent of several complete, fun, balanced single player games operating together in balanced harmony. Not impossible, but highly improbable.

It’s largely impossible now, it’s not a technical problem, it’s cultural.

UO forced many different types of players to coexist in the same world that simply do not mix anymore. You had peaceful dungeon crawlers and craftsmen coexisting alongside killers, rapists, thieves (wild that stealing items from other players inventory was actually a thing, probably unheard of in today’s MMOs).

The friction between these different types of players is where the magic happened, it’s what created real conflict and higher stakes in the world. When you stepped out of your house, there was always a risk that killers could be lurking ready to murder you and loot your house dry. And if you forget to lock the door, someone passing by will clean your house out for anything valuable.

In a way, old school UO was a true Middle Ages type MMO, everything since then has only grown more civilized, more enshittified. People don’t want to pay for a world that doesn’t give a shit if they have a bad experience. The truth is though there was no “bad experience”, it was all just an experience.