Several elements got me in, and kept me.
-Good environment for RP.
Enough background to have something to play off of, but unlike, say Lord of the Rings Online or any MMO set in the Star Wars franchise, you're not playing in someone else's story that you already know the ending to. Maybe you'll never be the Luke Skywalker of the story even in UO, but the point is that you don't really know that you won't, whereas in Star Wars that role is already fulfilled. And, unlike in a single-player game, you do not have to be the big hero or even to try. Hell you could have a full in-game life and never once touch the meta-fiction story arcs save for sometimes you might have to escape it if you unwittingly stumble into it.
The EM events only solidify this element for me.
-Right sandbox/directed experience mix.
One way to look at this element is the non-RPers version of the previous element.
Pure sandbox would be boring. Pure directed experience is a single-player game that happens to have some NPCs played by life people. UO's seen as a pure sandbox but it's really not,and that's a good thing.
You can have a full in-game life and never do a quest, or only take quests to get you into certain areas you otherwise wouldn't have been able to get into, never work an event, never do a storyline. Or you can fully immerse yourself in nearly-everything. The odds of fully immersing yourself in literally everything are small as there's simply too much to fully immerse yourself in.
-Trammel.
The last 2 elements got me in and kept me for awhile, but I'm one among many who aren't as vocal on these boards, who Trammel kept in the game when I was thinking of exploring Everquest. As UO was constituted before Trammel, it presented a pure sandbox for a select few and a directed experience to most (being e-mote ****d in the wilderness; smack talked to; house dry looted while a few dozen folks looked on and laughed, ready to gank you if you attacked the thief; etc.).
I simply never saw as many people just walking around, experiencing the world before Trammel as I did after Trammel. At some point, "can you help me get to my corpse" became much more likely to be a sincere request than an attempted lure.
-Galen's player