Thank you for proving that I'm not alone in this matter. A second reason for me to disregard the MQ is finding the words for unlocking the shouts. I have a lot, but have been looking for Fus Ro Da.
Bad luck has left me still with Fus. It's probably tied to the MQ, then. Many dungeons are very impressive, and I can't compare them to Oblivion because it would be cruel. And they are all scary.
Also, NPCs tend to make references to things that the game "assumed" you have already done.
Does that break the immersion for you main quest avoiders or does that not happen often in this game?
It hasn't happened at all. Scratch that, sometimes I get notes that say: "I was impressed with your performance at x. Please go to y dungeon, there is something good for you there" - Anonymous
But nothing specific is revealed. No one is going to say, "it's the hero of Kvatch! I'm glad to meat you," if you avoid the MQ.