“I know it when I see it”.
The most important aspect of the High Fantasy sub-genre is that the story occurs in a world which is not our own. I would argue that the terms High Fantasy and Epic Fantasy can be used fairly interchangeably.
Some of the best-known works of High Fantasy include The Lord of the Rings, Malazan Book of The Fallen, A Song of Ice and Fire, and The Wheel of Time
Features of a High Fantasy novel:
• An imaginary world (usually an entirely imaginary world, but can be a parallel world or a world-within-our-world)
• A battle of good versus evil or other epic theme, with terrible consequences if the hero fails
• Multi-volumed works with a complicated plot, a large cast of characters and an expansive timeframe
• Non-human races such as elves and dwarves and/or magical creatures such as dragons and unicorns and/or monsters such as orcs and trolls
• Medieval technology and feudal structures
High Fantasy novels can have all of these elements, or just a few.
BTW – I have been as broad as possible in my definition to make room for all novels I consider to be within the sub-genre. Of course, there will always be ambiguities and what one person may class as High Fantasy, another may claim is Romantic Fantasy etc. I will err on the side of a novel being High Fantasy if I think it feels right.
For a great mind-map of fantasy genres, see my post subgenres made easy