It's a good list, but, of course, there are many more authors who could have been included, so I'm taking a leaf out of Suddeth's book and posting my own list of ten authors who are either not generally thought of as speculative authors, or else aren't nearly well enough known to modern spec readers.
1) Chrétien de Troyes. Although Arthurian legend goes back further, it was the courtly poets of the 12th century who developed the familiar image of the Knights of the Round Table, forever going off on quests and rescuing damsels in distress. Chrétien was perhaps the best of these, treating the subject with a stunning mixture of wonder and realism, not to mention a considerable sense of humour.
Perhaps his greatest legacy is the creation of the Grail legend. Most of the elements pre-existed, but it was Chrétien who wove them into the tale of Perceval and his quest to discover the meaning of his vision in the castle of the Fisher King. Note, by the way, that the artefact wasn't at that time the Holy Grail, the Sangraal, or even a Grail-shaped beacon — it was "a grail", i.e. a serving dish, not a cup. Perhaps some of the people who write nonsense about the legend should try reading the original version.
2) Wu Ch'êng-ên. A 16th century poet and novelist, best remembered for his epic novel The Journey to the West, also known as Monkey. This tells the tale of the mythical Stone Monkey, and how he redeemed himself from his punishment by Heaven by joining a journey to India to bring back Buddhist scriptures for the Emperor.
The journey, undertaken by Tripitaka, Monkey, Pigsy and Sandy, is the best-known element and formed the basis of the classic TV series Monkey. My favourite section, though, is the early account of the Stone Monkey's origin, and his struggles with Heaven for dignity and independence.
3) William Shakespeare. Note, William Shakespeare, son of John Shakespeare of Stratford, possibly the greatest writer of all time — not some random aristocrat, dead dramatist or philosopher/scientist.
Shakespeare wrote plays of all kinds, but two of his most popular, A Midsummer Night's Dream and The Tempest, are unmistakably fantasy tales. In addition, two of his greatest tragedies, Hamlet and Macbeth, could be described as magic realism — essentially, political thrillers, in which supernatural elements play crucial roles in setting events in motion.
4) Samuel Taylor Coleridge. My favourite poet of all time, Coleridge wrote poems of philosophical speculation, natural observation and inspired lyricism, but he also wrote two masterpieces of gothic fantasy. Gothic novels were considered rather trashy at the time (Jane Austen's parody in Northanger Abbey is probably a fair enough picture) but Coleridge saw the possibilities for this sort of story.
The unfinished verse-tale Christabel is a brilliantly atmospheric story of an innocent heroine seduced and enchanted by a demoness, but The Rime of the Ancient Mariner is his masterpiece, with its account of the cursed ship and the tortured immortal Mariner. It's effective just read off the page, but far better either to declaim it aloud, or else to hear it. There's a superb recording by Richard Burton that's well worth seeking out.
5) Charles Dickens. What, Dickens a spec writer? Well, he certainly specialised in social realism (though some might argue that his plots contain so much coincidence that they might as well be driven by magic) but he wrote at least two classic ghost stories. The Signalman is a shivery short story (also notable for being the only time Dickens mentioned the railway, in spite of writing till the 1860s) and, of course, there's A Christmas Carol, with its succession of ghosts reforming the old miser.
6) George MacDonald. Victorian novelist, poet and liberal theologian, Macdonald included a number of fantasy romances among his work, notably the dream-fantasies Phantastes and Lilith. It's perhaps less known that he also wrote at least one work of science fiction, long before Verne and even longer before Wells. This is an episode in Phantastes, in which the hero reads a string of stories in a strange library. One is set on a planet which is a lot further than Earth from its sun and consequently has a much longer year, so that its inhabitants rarely experience more than one season. Not bad, for the 1850s.
7) William Morris. Better known as a poet, artist, socialist thinker and wallpaper designer, Morris was described by Lin Carter in the 1970s as "the man who invented fantasy". This isn't entirely accurate — Carter elsewhere rightly claimed that fantasy is the oldest form of literature — but Morris was possibly the first author who simply made up a world to set his stories in, because they wouldn't fit into any corner of the real world.
Having said that, Morris's world is very like an idealised version of mediaeval Europe (though his heroes and heroines are more likely to be peasants than knights and ladies) but the countries, cities and customs are all invented. Morris wrote in a very archaic style, which might take some getting into, but it's well worth making the effort. The Well at the World's End is probably his best — and very obviously Tolkien's model for the plot-outline of Lord of the Rings, although the stories' natures are very different.
8) Lord Dunsany. If Morris "invented" the modern fantasy novel, then Edward John Moreton Drax Plunkett, 18th Baron Dunsany "invented" the modern fantasy short story. He did write novels, notably The King of Elfland's Daughter, but his greatest legacy is in developing the standard forms of short fantasy. His tales range from exotic cities and gods to heroic adventures to magic intersecting the familiar world. Both directly and through authors he influenced (notably Lovecraft) he's had an immeasurable influence on fantasy.
9) James Branch Cabell. Mostly famous/notorious in the 1920s for having his novel Jurgen (unsuccessfully) prosecuted for obscenity, Cabell created perhaps the first complex mega-series in fantasy. The various components, linked by family ties and the idea that the characters were collectively playing out the comedy of mankind, ranged from an approximate version of mediaeval France to 20th century Virginia (in one case, in the same novel) and beyond into a variety of mythical realms.
Not all aspects of Cabell make comfortable reading today — his sexual politics, for instance, were, let's say, very much of their time — but he wrote with a huge sense of fun, mischief and wonder. The best novels to start on would probably be Figures of Earth (nominally the beginning of the series, although it's not quite as simple as that), Jurgen or The Cream of the Jest.
10) Hermann Hesse. The only Nobel laureate on my list (partly because many of the others predated the prize) Hesse was born German and naturalised Swiss. His work ranged through contemporary realism, historical fiction, magic realism, mystical allegory and science fiction.
Steppenwolf, perhaps his best-known book, is a psychological examination of its central character through a "magic theatre" in which he can live his fantasies. The Glass-Bead Game is a work of philosophical science fiction, set in the future where society's spiritual needs are expressed by the game of the title.
Restricting myself to ten, as in the original list, means I've had to leave out plenty of authors I might have included on my list. Hopefully, though, what I've helped to show is that not only are there many great speculative authors who aren't as well known to modern readers as they deserve, but speculative fiction is far more widespread and far more "respectable" than the literary establishment try to pretend. After all, if speculative fiction was good enough for Shakespeare and Dickens, who is some obscure academic to look down on it?