UO was one the buggiest online games ever made. There were so many problems: patch issues, they had to keep graphical compatibility between 2d and 3d clients, very bad game design problems, no proper testing of features etc.
Ultima Online did have a lot of bugs. Being fair to the developers, they were sailing in uncharted waters when developing Ultima Online, not only in the technical sense, but in the social sense too. Raph Koster goes into more detail about the interesting lessons they learned from their mistakes in designing Ultima Online's early gameplay. A lot of UO's bugs spawned interesting emergent behaviour from the playerbase too.
One of the "bugs" created a system of epic items. Rares.
Rare items were random objects that normally could not be obtained by players. Due to bugs, items that were only meant for decoration etc, were available to players.
For example a pair of shackles in a dungeons that were supposed to be locked down as a decoration item. On every server restart these shackles would become "unlocked" from the ground and players could grab them. They would then go to a grey market where there was a big trade for rare items.
There were some rare items that were only available once per server. Mind you, if a player lost them they were simply gone. One of the greatest rares was a piece of cloth with a rainbow pattern that wasn't available anywhere else in the game.
Even with all its bugs (which I agree, there are a lot), it was probably one of the greatest games ever made, especially considering the setting it was made in.
Still being praised, played, and respected with all its bugs is kind of a validation.
Duping and sigil dying were some note worthy defects, not to mention all the npc economy exploits -- all of which helped to destroy the retail game eventually.
