Discussion of Lovecraft’s work tends to focus on three things: racism, cosmic dread, and tentacles. However, as anyone who’s spoken to witnesses to an event knows, the same story becomes different depending on the author: so, while a tale of a Englishman who discovers a debased African tribe summoning a writhing horror and is traumatised by the realisation the universe isn’t designed for humans is immediately recognisable as Lovecraftian, what might distinguish Lovecraft’s version? While the exact combination of qualities might well require a theorem as long as his works put together, his core of his prose style isn’t that complex: Lovecraft’s sought to replicate a formal British style that was already considered archaic when he was writing and was more inclined to reportage than immersion.
His early works are an homage to authors whose work he loved, Lord Dunsany in particular, so are especially prone to an admixture of complex, precise sentence structure (rather than more casual, natural speech) and invented fantastical names for places and people.
This echoing of authors from a prior age fades but his love of the idea of Great Britain, of a respectable nation of educated (Protestant) aristocrats and ancient evils buried in the (pre-Christian) past, continued so his style remained deliberately more formal and archaic than the American English of many who were his peers.
Whether deliberate or unconscious, Lovecraft’s style also takes from his love of reason and dislike of religion, conjuring the distance of science rather than the rush of emotion. His narrators almost entirely describe what they perceive and discover rather than sharing what they feel; it is only when they encounter the things that they cannot bring within their rational model of the universe that they display emotion, but even then it is the swift collapse of a mind unset rather than a shared experience with the reader.
This archaism and focus on what can be narrated shape his story arcs overall, creating a long lead with little overt action followed by a sudden burst of horrific revelation at the end rather than the more modern story shape of several peaks of increasing size.
Lovecraft was not alone in this earnest copying the authors that first inspired him: many fans of the Mythos write their stories in Lovecraft’s style and with his concerns without a hint of comedy (in intent if not in result). A juvenalian (sic) act to which I also succumbed: I still recall one of my first forays into the depths of MS Word was finding a way to have it suggest ‘anent’ every time I typed ‘about’.