It was a incredible world to discover and explore (especially custom servers with custom maps and content). There was no 'point' to the game, i.e. no final destination or quest to complete... in fact there were barely any quests at all. It was all just up to you. However you wanted to play you could. I.e. you could just have a blast logging in and hanging out in a player's house which they converted into a bar and play chess chatting about w/e. I even played some great roleplaying shards which were legit, e.g. a LOTR one and it was soo epicly awesome. Good pvp dynamics. I did love the ruthlessness of losing everything when you died. I loved how custom shards could tweak so many of the variables pretty easily. I just have so many absolutely golden memories, hilarious and absurd memories. Idk... it's just really hard to convey how the game was the best. I think a lot of is just because it let you do whatever you wanted, but also provided you with fun PvE, PvP, roleplaying, etc.

What are games today missing? They're not persistent open-world sandbox games. Sure we have open-world games, but they're not sandbox and often not persistent, and we have sandbox but they're not open-world and often not persistent too.

I can't do it. I can't adequately describe what games today are missing which UO had. It's a quality of the combination of all the features UO had that just resulted in the best memories and fun I've had when gaming. I just don't see enough support for player-generated content as UO had, which I suspect is a big part of it too.