I quit UO in 2002 or 2003 when the ridiculousness of the expansions was starting to skyrocket. I wasn't exactly happy with skills going to 125, but it was still sort of playable. But when they screwed the classes up, added weird new worlds and stuff, that was the end of the line for me.
I also only started playing shortly after Trammel was introduced, so I didn't quite experience the "true" UO as the oldtimers were saying.
Needless to say, I had shitloads of fun in those two years.
One of the biggest downsides of the game was extremely shitty client with absolutely horrible performance.