Massively looks back at Ultima Online:

UO got one thing right: it felt like a persistent world where one could simply exist. No other MMO has ever been able to get that feel right, though Star Wars Galaxies came close. These days, I don’t feel like I’m an active participant in my MMO’s world; in Ultima Online, I felt that I had a real place in it.

That seems to be a common thread talking to ex UOers - you could just ‘be', without participating in endgame or designer mandated gameplay.