Tolkien's Lord of the Rings is the benchmark for adult High Fantasy, and its mark is seriousness and realism about the world depicted.
In other words, pure High Fantasy must not have the slightest hint, trace or taint of ironic detachment or allegory; certainly no parody or satire - that is absolutely fatal; nothing 'arch', no breaking the fourth wall, nothing post-modern; no nudge-nudge humour about the quaint ways or beliefs of the fantasy world...
Of course this means that many people will - and they do - hate High Fantasy, because they find it boring. And many other people read High Fantasy in the wrong spirit - they read it asif it was an allegory - they enjoy it only by subverting it.
By this test, examples of High Fantasy would include Lloyd Alexander's Prydain chronicles, Alan Garner's 'Weirdstone' and 'Gomrath' novels, and JK Rowling's Harry Potter series. It would also include Terry Pratchett's first three Tiffany Aching books.
Most of Fantasy literature is not High Fantasy by this test - for example most of Terry Pratchett's Discworld series is parodic and satirical in its humour. CS Lewis's Narnia books are not consistently High Fantasy either, because of elements of narrator commentary from a modern standpoint - for example, the mockery of Eustace Scrubb and his parents, or of progressive education. And Lewis's Space Trilogy is too allegorical to be High Fantasy.
But I suspect that this interpretation and distinction I am outlining is probably not universally accepted.
When I recently read Jonathan Strange and Mr Norrell by Susanna Clarke, and looked at the reviews, I came across reference to S&N's endorsement by Neil Gaiman, and how he regarded S&N as the 'finest work of English fantasy' written over the previous seventy years.
I certainly agree with the Gaiman's insightful appreciation of S&N's quality
- but I disagree with him about how to classify it.
By 'seventy' years, Gaiman was referencing a novel called Lud-in-the-mist by Hope Mirlees published in 1926. On this basis, I went to to read Mirlees's book, and found it both enjoyable and well written.
However, it was a qualitatively-different kind of book than Strange and Norrell. Lud-in-the-mist is not in the slightest degree believable; Lud is not about world-building or alternative reality - it is rather in the genre of a fin-de-siecle 'exquisite miniature' like Oscar Wilde's stories (e.g. the Selfish Giant). It is arch, satirical, self-consciously modern.
By contrast with Gaiman, I would regard Strange and Norrell as being in essentially the same category as The Lord of the Rings; for the seriousness, depth and realism of its alternative magical and fairy world. For the 'feel' of it.
There are also differences, but this similarity is the one by which I would choose to classify Strange and Norrell.
The thing is, S&N was marketed as a mainstream novel, not a fantasy - and the reviews and publicity focused on its pastiche literary style, as if it was a postmodern take on of Jane Austen - rather like John Fowles's once highly-rated The French Lieutenant’s Woman was a Victorian pastiche (and postmodern commentary).
But I would regard Strange and Norrell as true High Fantasy both in spirit and attainment; and I would therefore prefer if it had been marketed to the fantasy niche, and reviewed as such.
As such, I think S&N would have found more readers who took it with a seriousness appropriate to the book's scope, nature and ambition.