So many other people have already said many of the things I would put as my "top reasons" for preferring, and often returning, to UO and the unique things it offers.
One thing that simply amazes me what with how MANY different MMO's have been made is how almost no one else has even so much as considered the level-free design. The level-free design of UO means that while there are certainly places dangerous for newly created characters, there is almost no such thing as something in UO that friends can't do together. There is a massive (and often crippling) hidden mini-game in the design of virtually every other MMO (and let's face it.. all of them are basically copying WOW, which in and of itself was merely a more refined version of the Everquest formula of level, level, level, then raid, raid, raid) where you have to try to plan things around who is within very close level of you, very close gear progression of you, and if you plan to play with a friend you two practically better set an arrangement in stone and sign it in blood that you will only play together for the same amount of hours. Otherwise, you get so out of sync in terms of levels, gear and power that you rapidly find you can't do anything meaningful together in game at all. I remember in UO things like impromptu guild gatherings where every member who was online would just up and go to Shame or something, for kicks. New joins, old vets, well equipped players, poorly equipped ones, we'd all go together and just take care of the group. You can't do that kind of thing even if you want to in another MMO. They've sold out hardcore to the idea that the only possible reason any player would have to keep paying sub fees or keep playing their game is to get massive leaps in character power on an ongoing basis. It creates this horrible and endemic situation where there's a huge glut of players at max level or the top end of the powercurve, with everyone who joins late or is levelling a new character essentially has to solo the entire game and just rush through to catch up. It makes 99% of the total game content pointless wasteland.
Granted... UO did not entirely escape some of this design with the horrible changes surrounding AoS and bringing in a Blizzard design team on contract to "spice up" the game with the stupid stat gear and powerscrolls and everything. But UO remains a game where you can pick up your friends and always go do something together, no matter how they're built or how much they've played. You simply cannot do that in virtually any other MMO due to level and gear differences translating into astronomical differences in power levels.
and my aol internet seem fast 
