I'd like to propose a slight increase in favor rewards for completing quests on Epic difficulty, something along the lines of (using Heroic as a baseline) +1 for successful completion on Epic Normal, +2 on Epic Hard and +3 or +4 on Epic Elite.
There are certainly less than 100 Epic quests in the game, so we're not talking about a ton of extra favor to be earned. It just seems to me that, to be post-level 20 and go back and do a low level quest on Heroic Elite and get the same favor reward as doing a quest on Epic Elite, possibly under level, just ain't quite right. Give players sufficient reason to run the hardest difficulty and they'll do it.