I was once a slush reader for a SFF magazine and also for a small Canadian press. I saw a lot of epic fantasy. There were the 350k tomes and the cheesy sword and sorcery short stories. Epic fantasy can be a hard sell these days. Many readers are tired of the same ol' that's out there. They want something fresh.
Where is your epic fantasy set? A large amount of epic fantasy is written in a quasi-medieval European or Britain. If your book is in this setting, what distinguishes it from all of the others on the market today? Likewise, some writers are moving their fantasy to an Asian setting. Again, what is setting yours apart?
If you have a made-up setting, does your culture logically evolve from the setting? How about the religion? Clothing choices?
Is your story sexist? There are two parts to this question. First, your story may be sexist on purpose. You might want to create a world were women aren't able to fight in wars or hold positions of authority. That's fine. The question is how do you address this issue. Do you have women exercising great power from within their own spheres? Do you have women pushing the boundaries whenever possible? Do you have women supporting the status quo? Or, is the only rebel against the system the lone female archer mercenary who drinks and swears and sleeps around?
Second, look at your cast. Are all of your female characters in positions of minor importance to the plot? Do you have any major female characters? Do you force your women into arranged marriages, even though this isn't a standard cultural action for your world? Do women always have to be rescued by men? Do your women know nothing about midwifery, including abortion? Are women raped because they were objects of lust and the men couldn't control themselves?
Do you have Stuff People Skip? Tolkien may have written pages of description but that does not mean today's writers have that luxury. Literature is fluid, ever changing. Many of today's readers aren't interested in pages of description where every single item in a room is described. It becomes "stuff people skip." Many epic fantasy tomes that I saw in the slush were that way because of the unnecessary description.
Are you trying to write another Lord of the Rings? Tolkien already wrote it, sorry. Same goes for anything by Terry Brooks or Robert Jordan.
Does everyone believe in the same faith in your book? One world religion? How does that work? Likewise, is everyone atheist? So, no one believes in something other? Again, how did that come about?
Does your magic system have consequences? Do you even have a set system? What prevents people from using magic all of the time? Can your plot pass the "cell phone" test? (cell phone test is "can this be prevented if the main character carried a cell phone." In fantasy, it's "can this be prevented if the main character uses their magic?")
These are just a few tips to help you look at your manuscript objectively. Please share some other things that people should be on the watch for in fantasy!