UO is a sandbox game, it's limit is your imagination. UO doesn't have an endgame because it has so much freedom, you can do whatever you want really, the game is what you make of it. For me UO is a never ending game for me because of roleplay, the possibilities are endless when it comes to that, the storylines are constantly changing etc etc.
You can attend the EM events which is a constant ongoing storyarc and side stories that are never ending, you can become a trader and set up a great shop, you can pvp in fel if you want or PvM to get phat lootz, you can spend your days customizing great houses which some people are very found of, you can have the hobby of collecting rares and items and creating a museum. You can create great events for yourself and others such as auctions, the chicken league, bagball etc. You can set up market stalls and build stuff by placing things on the ground.
No other game has ever come close to this sort of Freedom apart from Eve Online, but spaceships aren't everyones cup of tea and still the freedom there isnt as great as UO i don't think.
