This question probably only applies to you if you read a lot of fiction, particularly fantasy. When I finish a book, I think about it. I do a lot of thinking. I roleplay in my head, things like that.
Some books, when I visualize their world, it looks like a really well drawn picture, but a picture. These are good books, but they don't appeal to my sense of 'epic'.
Other books, when I visualize, I get a 3-D image, usually at night (I love night), and everything's nice and shiny and beautiful looking. These are books that I describe as epic, because I have this emotion that I don't believe most people feel often, I call it my sense of epicness.