In fact, my overall impression is uncertainty, and that's also my reaction to the novel. While not exactly a Lovecraftian story, at the same time, it's almost quintessentially Lovecraftian. It features impossible geometries, characters who's lives are broken by their encounters with "the Mythos" (including two suicides, several other suspicious deaths, a descent in alcoholism, and the book's final scene is set in an insane asylum), horrors that predate humanity by hundreds of millions of years (and seem to be chronologically associated with various trilobites; the first time I've ever heard of them being used as an icon of horror) and seem to come from elsewhere than our earth.
Yet despite all this, like Total Recall or Inception, at the end of it, you wonder if perhaps the whole thing was a hallucination of characters who's elevator don't go all the way to the top floor, if you know what I mean. There's little that's explained satisfactorily, no evidence left behind to corroborate the beliefs of the characters, and perhaps most frustrating of all, a seminal plot point that takes place in the prologue is never actually described; characters refer to it obliquely while Kiernan refuses to ever tell us what happened to them exactly; both to the frustration of some other characters and to the readers.
I've said before and probably will again, that there's a fine line between holding back information about your monsters and revealing them. Reveal them too soon and too clearly and you manage to deflate much of their ability to actually cause any fear; refrain from doing so at all, and you've managed to pull a bait and switch on your audience, who comes to supernatural horror fiction precisely because they want to be exposed to supernatural monsters, and they want to see them and understand them, and be goggling at how cool a concept they are. Lovecraft himself frequently botched this execution; he overexposed some monsters (crinoid Elder things most spectacularly), never really exposed others at all, so that we're not even really sure that they're supposed to be scary or not (whatever's at Kadath in At the Mountains of Madness) and frequently revealed creatures that were underwhelming when their big reveal actually happens (ironically, Cthulhu itself, and shoggoths, among others.) Kiernan both does and does not fall into this trap--her horrors are shown and described, but never explained, and their nature is nearly as mysterious at the end of the book as it was at the beginning. And much like with Lovecraft's At the Mountains of Madness, I think the end suffered from rushing breathlessly through a conclusion that is vague and suggestive without actually satisfying the readers very much.
One thing I'll give Kiernan--we're never supposed to believe that Threshold is scary because she tells us that it is--she very effectively does cultivate an atmosphere of dread. Well; two things--although I'm still not sure that I even liked this book, I can't deny that it lodged itself in my brain quite well and inspired an awful lot of thinking about things; about Lovecraftian horror, how to do it correctly, about weird extradimensional creatures outside of time and space and what their relationship to humanity might or might not be, and about freaky teenaged girls that might be heroes of a secret war that most of humanity is protected from for its own good, or might be merely insane, psychotic murderers. While I don't know that any fan of Lovecraftian horror would necessarily like this book just like I didn't necessarily like it, I'm confident that any fan of Lovecraftan horror will find it fascinating nonetheless.