What i think is the single most important aspect of a truly epic questline, is variety. By that I mean choice. It can be a great story, but most questlines are just an...electronic book.
I want choice, in dialog. I want those choices to be available pending my character's... Race Gender Skills Accomplishments of my character thus far Their clothes and or armor The weapon they are carrying What they have in their current inventory Reputation The dialog choices made thus far in a conversation
I want choice on how to start a quest.
I want choice on who to side with during a quest.
I want choice in how to complete a quest.
I want multiple branches that will lead to multiple endings.
If these elements are not implemented, you are just going through the motions of reading an electronic book. One that has the pages turned by the exhaustion of dialog. Which, in turn, has no affect on the story one way or the other. That is not dialog choice. It's dialog advancement.
I want choice and consequence.