I believe "Shadow Out of Time" was written after this one. In that case, this would, I suppose, be the first Lovecraft story wherein it's as if the presiding genius is Olaf Stapledon rather than Poe, Machen, or Dunsany. In HPL's more typical stories, the alien civilizations and bizarre entities did not, I suppose, necessarily have very much behind them; for the most part, it seems they were there for the sake of the weird atmosphere and horror effects that HPL wanted to conjure. Here Lovecraft is writing an imaginary-realm story and proposing quite detailed "knowledge" of an alien civilization more, it seems, for their own sakes, though within the framework of a weird horror story.
Tolkien was the greater writer, but I think something similar happened in his case. he wrote the stories he is best known for, and in them, although he was, to be sure, able to draw on an existing mythology (that of the Silmarillion) that was unknown to nearly all of his readers -- still there was also a freedom of improvisation for the sake of adventure and atmosphere, e.g. with the Ents, etc. But in his late work (in The History of Middle-earth) one sees him querying his own imaginative creations and looking to systematize his "knowledge" of them. Was HPL perhaps doing something of the sort? In his last few years was he, indeed, really developing an elaborate "mythos" that might eventually have run to hundreds of pages of imaginary worlds, histories, races, etc.? I wonder. But if so, might he also have found that he lacked the desire to tell stories -- or at least that it was a weaker desire than it had been?
"The Colour Out of Space," which I commented on, at length, over in the Groff Conklin anthologies thread a few days ago, is a real work of art. Pretty much everything all works together.
In Mountains, though, I keep getting a sense of things pulling different ways. If engendering suspense, etc. is the purpose (clearly it is a purpose), then some material is pretty questionable in terms of letting the cat out of the bag, etc. For example (quoting from the Penguin edition of The Thing on the Doorstep &c), on page 263, an allusion to "Elder-Things supposed to have created all earth-life as jest or mistake," and again, on p. 266, "Great Old Ones who ... concocted earth-life as a joke or mistake." On p. 286 there is more of the material alluding prosaically to R'lyeh, etc. See my message #30 above for why I question this.
Very well, then let's suppose HPL largely wants to invent a really detailed ancient realm, a lost race scenario to rank with the greatest -- he is going to have to struggle with how to present it, and of course does so in the form habitual to him, the horror story. And so you get this phenomenally protracted, delayed expository matter to lead up to the great shattering moment, which readers sometimes feel doesn't succeed as "pay-off." Well, then the story is criticized as horror story clogged with way too much detail. What I'm saying is that maybe one could criticize instead the decision to cast the story as a horror story, a sequence leading up to one shattering moment. Might it have worked better if he'd cast it as an admittedly episodic adventure story with, to be sure, a strong element of the weird and horrible? Add some Rider Haggard elements? -- by which I don't necessarily mean a love story!