The freedom...
House design,deco, storage. I may be a poor apartment renting college student IRL, but in game I can be a castle owning, veteran, pack rat. I can not only build a house the way I want to, but even with castles I can deco them exactly as I want to. There are so many items in this game that with a little creativity and vision I can do amazing things. Sure "Tile" based worlds may be a little out of date, but man it provides endless possibilities when it comes to house deco. I love seeing other peoples house designs and layouts. Some of them are the ugliest most hideous things I have ever seen, and they make me feel so much better about my own design. Others are simply beautiful, and show me whats still possible and how much I can improve on. The intricate, detailed, customizable housing is probably the biggest reason I play.
Skills: I can mix and match any skills in any amount that I want to. I have tried a lot of games, and nothing is as cool as making my own character template. Sure, some templates work best for certain situations... but in UO i have the CHOICE to use those templates, or try one of my own. Easily the second biggest reason I play UO.
Gameplay: I can PvP, PvM, play in groups or alone, craft, garden, collect rares, and buy and sell anything I want. I can fish, I can hunt for treasure, I can tame and train animals. I have never been into roll-playing, but if I decide i want to try it, this is the perfect game to do it in. I can play whatever kind of character, and do what ever I want to do any day of the week. There are countless options and I am not forced to do a single one of them if I dont want to.
Its also kind of cool to log in and play with people age 10-75, nearly all of whom can each teach me about some aspect of UO I still don't know about after 125+ months.
Some of this just comes with time... but you can bet I will never even try another game unless it offers customizable housing, and skill freedom.