I guess that's part of the difference. I enjoyed the puzzles and challenges in MM1 tremendously, and eventually, they led you to complete the main line quest. I don't mind forgetting about the main quest as long as the others continue to push me toward it without me realizing it. If they're just busywork, I don't like 'em. So a lot of quests that don't do a whole lot to advance the story... that I don't care for.
Things like the priest promotion in MM6 -- rebuild a temple? And there's no temple there until you do rebuild? Now *that* is a way to tie things in
And as I'm playing through MM6 now... I really enjoy the number of dungeons per map. There seem to be three or four per map, which leaves lots of room to explore. MM9, I think it was one or two per map -- once you're there, you're done.