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One word answer to the thread title: Community!
UO was the very first MMO I played and it continues to provide so many elements that are found lacking in newer games.
More than that, the community that remains, and I admit that it is not what it once was, are dedicated to ensuring the continued success of the oldest MMO still running.
15 years of history are hard to underestimate, the love for the game is seldom seen elsewhere.
Take a look at what one group of players have dedicated their time and effort to produce for this community: