I thought of my novel as high fantasy (though not necessarily epic fantasy) before I even started writing it. Although the world is basically Earth like (it's round, follows most of the laws of physics and biogeography), it is clearly separate from ours, has its own history and cultures, a number of species that don't exist in ours (though no elves or dwarves), and magic plays a prominent role in the story.
Now that I'm at the stage where I'm starting to formulate agent queries, I've tossed a few samples I've written up in fantasy writers' forums. One reader commented that there was nothing in my query that made him think my novel really was high fantasy. Well, a query has to focus on the plot and characterization, not world building. When I told him this, he said that doesn't matter, because high fantasy isn't about the world building or conflicts, but about the voice and tone of the novel. This engendered much disagreement and a lively discussion about what HF "really" is. Some of the (competing and complementary) definitions people tossed out:
--there has to be elves, dragons, dwarves etc.
-- high fantasy is concerned with world-level wars and conflicts
--there need to be active supernatural entities involved in the story (aka gods)
--the laws of physics and nature must be very different from the real world (I guess that means a flat world, or floating cities that must be reached by winged mounts)
--the setting has to be largely medieval, both in terms of culture and technology
--the setting should be pre-industrial, though not necessarily medieval
--the struggle between good and evil must be central
--the tale needs to involve some kind of heroic quest that takes the characters to various locales within their world
--the main protagonist needs to be larger than life in some way, and preferably royal
--destiny needs to play a major role in the story (aka the protagonist is a chosen one)
--magic must be central to the plot
--there need to be at least some sort of non-human race or culture
My story fits some of these, but not others. When I mentioned my confusion over this on a different fantasy writer's forum, a fan of my novel commented that she didn't think it was HF because my magic system was too scientific and my healing too advanced (the nature of my world's magic allows healers to visualize and manipulate the workings of the body to some extent, so they're not mucking around with the degree of ignorance exhibited by "real-life" Medieval and Renaissance era physicians, nor are they merely waving their hands and allowing a god to do the "real" healing).
In desperation, I turned to wikipedia, and found a very broad definition of HF.
"High fantasy is defined as fantasy fiction set in an alternative, entirely fictional ("secondary") world, rather than the real, or "primary" world. The secondary world is usually internally consistent but its rules differ in some way(s) from those of the primary world." (http://en.wikipedia.org/wiki/High_fantasy)
In addition to well known examples, such as Tolkien's and Robert Jordan's works, the site included novels by authors such as J.K. Rowling, Piers Anthony and Phillip Pullman as examples of the genre.
By this definition, my novel is probably High Fantasy.
But a quick perusal of other web sites that define/discuss HF lead to more confusion. Some people claim high fantasy is the same as epic or heroic fantasy (http://www.ghostwoods.com/2010/01/high-fantasy-1000/), while others make a distinction (http://fantasy-faction.com/2011/survey-of-fantasy-subgenres). Adding to my confusion are the plethora of different definitions of the terms "low fantasy," "heroic fantasy," and "sword and sorcery."
According to Fantasy Faction's entry on the subject (http://fantasy-faction.com/2011/survey-of-fantasy-subgenres), George RR Martin's work is epic fantasy, but not high (not sure why not). If they are correct, my novel would not be high fantasy. And there seems to be some disagreement about whether high and low fantasy are differentiated by the setting, the role magic plays in the story or the nobility of the characters (http://writerstevensymes.blogspot.com/2011/10/high-vs-low-fantasy.html ; http://ultimatedm.com/?p=968 ; http://bestfantasybooks.com/fantasy-genre.php).
I guess the take-home message here is that there is no consensus about what high fantasy, or the other fantasy subgenres, really are. Some definitions exclude even classic works usually cited as examples of the genre, such as Lord of the Rings, from the category of HF.
I suppose I can just query my novel as a work of fantasy and let an agent or editor define it. Most bookstores lump all adult fantasy and SF together anyway, and many agents simply say they take fantasy, without defining specific subgenres.
My main concern is that I've researched a few agents who say they take SF and fantasy but stipulate, "No high fantasy." Since there is no "set" definition of what HF is, I'm not clear whether they'd be interested in my MS or not. If they are simply saying they don't like works set in faux-medieval societies with stereotyped fantasy races and tropes (told in that old-fashioned, stilted language), then I'm probably okay. But if they're saying they don't want anything taking place in a separate world with its own history where magic plays an important role, then I'm not.
I guess there's no harm in submitting when in doubt. I'm going to rack up a ton of rejections, no matter what. It's the nature of the business.