There happens to be one "main quest line", but like any of the other major questlines in the game, if you focus solely on it to the exclusion of everything else, you'll finish it very quickly.
Finished the main quest and the "guild" quests and ended at level 48.
I've started again as a Breton Thief/Assasin with Restoration (It's such a need on Master >.>) on Master and i have to say that i'm almost 30 and didn't done to many important quests. Master it's so fun and challenging.
' Wrote:it's actualy the clavicus vile daedric quest. it was pretty fun too.
That dog was fun for a few minutes, but I ended up getting annoyed at how it would constantly push me around--unlike the human followers it didn't stop at a respectful distance and would just keep walking into me.