I began playing UO around 9 years ago, and stopped not long after Trammel came out for the same reasons thousands of others stopped.
What may have kept many of us playing would have been for the devs to implement a few pvp shards that had the same basic pvp rules prior to the Trammel expansion. Offer a free, time-limited character transfer to the shard so that the pvp shards would be immediately active after implementation. Such an amazingly simple concept, one that was voiced for years, but shamingly, has never been acted upon. (Siege Perilous is not what I had in mind).
In addition, since the devs decided to make PVE their highest priority at that time, they should have implemented patches (or expansions) that greatly improved on the mob AI and game graphics immediately after making that choice. Of course, they chose not to do either of those things in a timely manner. Had they done this, UO might have had a snowball's chance at competing with WoW which, of course, is heavy on graphics and monster (boss) AI.
However, even the relatively well-designed games such as WoW become stale after a few years. I figured I'd check out the forums at what used to be my favorite website back in the day, and learn about the state of the union of what used to be my favorite game. Pre-trammel UO was, and still is, the best game I've ever played. Considering what I've been reading on the forums, however, I don't think I'll be reactivating my account any time soon.