Thing is that UO wasnt a grind, it took months and months to max a skill yes, but it didnt feel like grinding (at least not to me). It took me almost a year to get my first max skill, I didnt grind it.. I simply played the game normally and the skill progressed as I played. I probably could have sat there and grinded but there wasnt much point, I simply played the game and levelled the skill "naturally". At no point was I locked out of content because I hadnt "kept up" with more intensive players.
This is in a way, a fundamental change in the kinds of people who play MMOs, today that wouldnt be the case. A lot of players these days in MMOs are all about "firsts", or getting things as quickly as possible, in many ways the grinds are created not by the games but by the gamers insisting that they must get x , Y or Z as soon as possible, by doing so creating their own grind. In a way they have forgotten how to play just for fun and have focused on playing for attainment instead.
I actually think its one of the advantages of sandbox MMOs over themepark MMOs, for me at least the sandbox MMO has been simply about playing the game in whatever way , manner and pace you want, whereas the themepark MMO is often been about trying to "keep" up with other people so that you can join them on quests/raids etc due to level differences and so on, the focus being on the next character level or getting the next dps item or the next coloured item. You log in over and over because you are worried that if you dont for several days everyone you know will be X levels ahead of you, or at a different "stage" of the game and you cant do stuff with them anymore. For me, Sandbox MMOs have been about play, play, play and themepark MMOs have been about chase, chase, chase. Though thats perhaps a whole different debate.