I generally like to enjoy each world to the fullest. I'm paying for it, after all. So I like to do pretty much everything a world has to offer.
That said, I find I generally don't enjoy the long grinding dungeons very much. I play the game for leisure, so I want to have a fun easy-going time of it. When quests force me into long, tedious, multiple-boss dungeons with cheats and such, then I tend not to repeat those quests except when they are absolutely unavoidable to continue the main storyline. And I wish they were not in the main storyline too...
Main line only, pretty much. The only sides i bother with are the ones that give a spell or an important ability, and even then not on all wizards. The cost to do some of these by area is just too much to justify the garbage we get when we finish it. More xp? Okay, I guess. Wait no I'm trying to stay qualified for magus pvp, forget it.
Agree with those who say they need to improve side quest rewards DRAMATICALLY before a lot of us will start picking them up in droves. I have a job and family outside of the game that need my attention, too!