I think Age of Shadows expansion imo was the start of the downfall for UO. I played a Red on UO for a long time, and when Trammel came - I had a second character that was blue. Didn't phase me a bit... I loved the game for a long time, but switched to WoW because I needed updated graphics to stare at, and the "3D" graphics on UO were terrible. Wish they had come out with UO2. =(
WoW is still doing it for me. I still play it and have played it since Beta. I got a bit bored with the pull and AoE in WOTLK, but love the dungeons in Cata... Not a huge PvP fan in WOW. As it had the same problem that was added in Age of Shadows in UO - Too much reliance on gear. I remember people PvPing in cloth gear or practically naked in UO.
Played tons of others. Of course SWG - pre-NGE, tons of fun to be had there.