For Epic Fantasy there's a Cast of Character issue. Epics have lengthy casts. That's just how it goes. When you fall in love with Epic fantasy, as a reader, the complex casts and the character interactions on the backdrop of fully formed world become mundane. You wade through them, you have a method of keeping track of important points and the tropes of the genre help you to identify what specifically is important to remember.
When an aspiring author launches herself into writing Epic Fantasy, she *ahem*
Then, you start running into problems. You start hearing that you should limit your POV characters. You start hearing that readers should have a solid understanding of where the plot is headed by the third chapter.
You look at your library and think "But they didn't."
Rules for an aspiring author and for an established author are completely different. Brandon Sanderson is a good example of this. His brilliant Epic, "Way of Kings" was the first thing he wrote, but he couldn't publish it until he was successful with the Mistborn series and had been picked up to complete the Wheel of Time series. As an aspiring author--no matter how brilliant and wonderful his epic was--it couldn't land him with a publisher. He had do that with "smaller" books with fewer POV characters and simpler premises.
There are a few exceptions, of course, there always are.
But limiting your manuscript is, in my opinion, a necessary evil for most aspiring Epic Fantasy authors. It was also something that for years I could not figure out how to do. In fact, this draft of Silver Mask is what really taught me how to limit the plot and focus on the characters.
What is your favorite Epic Fantasy novel/series and why?