I'm pretty sure UO is (or was) unique among MMOs for having non-instanced player-built housing with full customization and decoration [1]. I fondly remember player-owned houses set up in high traffic locations such as near NPC cities, around moongates, along roads, and outside dungeons. It was so cool how you could go into a dungeon and fight monsters for treasure and then come out and visit another player's house which was set up as a shop, allowing you to restock on resources before heading back into the dungeon.

All of these houses and shops were customized and decorated to an incredible degree, giving the world a very cozy and lived-in feel. I know Star Wars Galaxies had player-owned housing but from what I remember the interiors of the houses were instanced away from the main world. This totally destroyed immersion. I can't tell you how many times I was playing UO, crafting in my house, when I'd see people outside wander by and step out to chat with them. Incredible stuff!

[1] https://duckduckgo.com/?q=uo+house+decorations&t=osx&iax=ima...