Ultima Online really does get mentioned a lot more than people think. There is an article up on Gamasutra, “RPGs and Suckage” and it looks at things that define RPGs.
Some of the defining characteristics that are mentioned include Loot, XP, Classes, Story, Acting, Turn-based Battles, Party, Lone Hero, and Exploration, the writer assigning loot, story, party, and exploration to UO. I would toss in “lone hero” though as plenty of people do a lot of soloing within UO and always have from the day UO went live, but within the context of the article, it probably wouldn’t fit.
UO’s level-less system, which has always been one of the strongest things going for UO, is mentioned:
First, there’s experience points and levels. This can’t be the defining characteristic, as plenty of RPG’s do without them – Shadowrun and Ultima Online being great examples. As for story, Dungons & Dragons can be run as a mechanics-only hack n’ slash campaign, and procedural games like Nethack have little to no author-imposed narrative whatsoever.