In all honesty, the story itself isn't tremendously engrossing in and of itself. What makes its entirely fascinating is its historical importance.
There are a lot of important things I don't really get. Try as I might, I can't find myself enjoying more than about ten percent of the Beatles' output. I get that they revolutionised music, but the problem with creating a new form of art is that what you sketched out, others can fill in. What you assembled, they can work on and polish.
Lovecraft is a similar case. Where the Beatles have "Penny Lane" and "Here Comes The Sun", Lovecraft has Cthulhu and Nyarlathotep. His vision of a insane, hostile cosmos in which the laws of physics are mutable and ever-shifting was chillingly brilliant, and made all the more effective by his considerations of the psychological damage his horrors would wreak. On the other hand, he had almost no interest in characterisation, and his stories all too often fit same pattern - the lead character begins an investigation into something freaky and gets drawn deeper and deeper until they die, go mad, or go mad and die.
It seems to me then that there might be more to gained by studying Lovecraft's world and methods than there might be in reading the stories strictly from the perspective of horror fiction. To return to the earlier analogy, horror writers have had over eighty years to build and polish.
Indeed, the foreword alludes to this approach by pointing out how much of the Cthulhu Mythos is born or referenced in the story: Cthulhu; the Elder Things and their Shoggoths; the Mi-Go; giant sightless subterranean albino penguins.
I guess that last one never really caught on,
The blurb is right, of course, ... Mountains... does indeed contain a great deal of sketchy information about Lovecraft's universe (albeit extrapolated rather unconvincingly from a series of pictograms). In truth, though, that's not really what's interesting here. What's really fascinating is not how the story ties in with the history of Lovecraft's world, but how it ties in with the history of ours.
I had no idea just how important this book was to modern-day horror. An expedition into the unknown? A distant research base that stops broadcasting after finding something horrific? A relief mission that finds everyone dead and falls foul of the cause? There are dozens of stories that follow this template, from Aliens to Event Horizon to episodes of The X-Files and Stargate SG-1 (it's also a favourite plot in video games, too). Aspects of the template appear even in non-horror films - check out Kirk's search of the Regulus Research Station in Star Trek II.
Given the time it took for Lovecraft's work to become well-known, it could be plausibly argued that this widespread dissemination of the idea originated not from the story itself, but from another source: John Carpenter's The Thing.
The similarities between the two are striking. Set in Antarctica: check. One research team goes off to check another team's base: check. The latter team finds an alien frozen in the ice: check. The alien comes to life and starts massacring people: check. The surviving team realises that they cannot let what they've found get to civilisation: check. Hell, even the idea that the dogs have a much better idea of what's going on than their masters do is in there.
In short, Carpenter (an acknowledged Lovecraft fan, who went so far as to call one of his films At The Mouth of Madness) could have called The Thing an adaptation of Lovecraft's novella, and the biggest issue fans of the original would be liable to have is how little the shape-shifting alien looks like an Elder Thing. That, or the fact that the bizarre and unsettling alien city was replaced by a flying saucer.
Also, what happened to the penguins? What is the point of that story without the penguins?
|"ARGH! I'm totally blind!"|
 Though of course they were new at the time. In fact, there is one implicit idea in there that I hadn't really thought about before; the idea that the Elder Things are so alien (barrel chested flying creatures with tentacles for legs, starfish for heads, and rotational symmetry order five) that they can't tell the difference between humans and dogs (hairy quadrupeds with two eyes, two ears, a nose and a mouth, though with very variable tail length), to the point that following their autopsies of the slaughtered camp inhabitants they actually accidentally put human and canine parts together when they try to reassemble their victims.