I played Dialbo II for *years--* I think I can safely say III won't have anywhere near that sort of longevity for me. Don't know if it's because I didn't enjoy it as much as II or if it's just because I'm a different person. I'll keep it on my hard drive though, just in case the bug bites me sometime in the future.
Also, I know I already listed my five favorite RPGs, but I wanted to make special note of my favorite MMORPG of all-time: Final Fantasy XI. I stopped playing it two or three years back because I no longer had the time for it (though I'm considering starting completely anew on it--so much new content since I've stopped playing!) It's got a very small following compared to many of the other big-name MMOs out there, and in a lot of respects it has not aged very well at all (which one can blame at least partially on the fact that all new expansions and updates still were required to work on the PS2--which yes, a lot of people still played the game on), but it had the best community of any online game I've ever played, and the amount of content in the game is simply staggering. I'm fairly certain that no other MMO out there has such a diverse range of activities for players to participate in (especially once you move into the endgame), and while a lot of these activities are fairly complex (maybe even clunky) they encourage teamwork and planning in ways I've never seen another MMO approach. The game had its fair share of problems (so much grinding @.@), but it was a genuinely difficult, genuinely vast game that you could play for years and still never experience everything the game has to offer. Even after several years of playing it I was still experiencing new areas and content for the first time, and was still learning the ropes with new jobs (classes). When I left it the game was sort of going through a crisis of sorts--there weren't a whole lot of new players anymore, which means that the developers had to restructure some elements of the leveling system in order to make it possible for players to continue to level up new jobs--soloing in the game is extremely difficult. So new mechanics were added to make soloing or playing with smaller groups of players easier, as well as allowing high level characters to 'sync' their level down to low-level characters and still earn experience toward their level by helping low-levels progress. I don't know what sort of state the game is in now, but with a major expansion coming out in a couple of months, it can't be too bad!