I realize this question doesn’t apply to all MMOs: Some MMOs don’t have quests at all, while in others, there’s no way you’re leveling without them even if you hate them. So adjust the question for your experiences accordingly.
But I’ve been playing LOTRO lately, and all I’m doing is sidequests. I’m quite certain that skipping large parts of the formal book-and-chapter storyline is a bad move, but I’m also leveling crazy fast without it, so I decided not to worry about it overly much until I have no choice. But even then, leveling as a VIP is so crazy fast that quests are going light blue long before I finish the actual area. (I know there’s a per-character cash shop item to slow down experience gain. I will not be paying real money for that.) In other words, I am leaving zones with a lot of sidequests done – and many more not touched at all.
Do you do side quests in your MMOs? How much of your leveling gameplay do they represent?