Re-reading those old posts got me thinking about the "Middle-earth in the style of Robert E. Howard" idea, and with some time to read in travel, I was thinking about the similarities and differences in Tolkien's and Howard's approaches to magic. The comparisons are interesting, and I don't think they would be difficult to fuse to a degree.
Compared to modern fantasy literature or rpg fantasy, both the Hyborian Age and Middle-earth are decidedly what we might term "low magic," which is not to say there is little magic in them. In fact, both worlds are full of things we would consider magical in the real world sense. There are any number of specially wrought items and substances that in D&D would be "magic items." Magic-users are not necessarily less powerful either, but they tend to use magic less and in less flashy--and certainly less "zappy" ways--than the D&D standard.
In Howard, you could say spellcasters are thinner on the ground. In Tolkien, that's true to an even greater degree; there are only like 6 wizards! But that's ignoring the special (magical) abilities so many people seem to evidence: the abilities of elves and dwarves to craft magical items, Bard and other Men of Dale having the ability to speak with thrushes, etc.
In an unsent letter, Tolkien addressed magic in LotR, drawing a distinction between magia (physical magic) and goeteia (charms, enchantments). For elves and spirits both of these are entirely naturally parts of the world, it's only the mortal races that view them as magic. Tolkien notes there do not rely on spells or "lore," and that humans can't perform them. This letter was unsent, though, and this last part contradicts elements of published works. The Hobbit speaks of dwarves casting spells (though maybe this is just superstition on their part and doesn't work), and even in the margins of the letter Tolkien reminds himself about Numenoreans using spells in making swords.
While Howard has the trappings of classic Sword & Sorcery spellcraft with summoned demons and dark, magical tomes, there is also an element of the psychic to his portrayal. In "People of the Black Circle" it's implied that belief plays a role in susceptibility to magic, even when it seems to be manifesting as physical phenomena, and that a lot of it's effect is hypnotism. Both Thoth-Amon and Xaltotun seem to accomplish a lot merely by directing mental energy without spell or obvious ritual.
For a more Sword & Sorcery Middle-earth, it goes without saying that Morgoth and Sauron, at least, taught sorcery to mankind. Sorcery that arises from evil and risks corruptions fits in well with a Howardian vibe (though, as I've mentioned before, not all spellcasters in Howard are evil. Just most of them!) Also, I would also have evil magic-users (Sauron, Saruman, etc.) perform more magic and more visual magic than in LOTR as written, more along the lines of things we see in Hour of the Dragon--where interestingly, keeping a magic item of power out of the hands of an ancient, awakened evil out to conquer the world is the key to that evil's defeat.