Seeing that most posters that have my point said that the biggest problem is players basically tiring themselves out going hours and hours into a battle.
Now this might sound a little unhealthy... but that's exactly what I was going after. Sure even now taking hundreds of pulls on a ~10 minute fight adds up to many hours, but when you get that kill after that 10 minutes... sure it feels great, but it feels great in a wrong way. It feels great that you got that repetitive 10min parse completed, and you're already tired of ever seeing it again. There is no feel of epicness, but more of just relief.
But imagine going hours into a single boss battle that never repeats itself during the time, every phase is something new, cool and difficult.
Just having something like this in the game would make the game much more epic.