The Dark Descent is a peculiarly difficult anthology to review because if it had a different title and reason-to-be, I'd be a lot less judgmental about it. Let's say the title is '60 Horror Short Stories and Novellas that David Hartwell Really Enjoyed.' Fine. I can get fully behind that anthology. So if that anthology were this anthology, contents unchanged, I give it a solid A- and request that in subsequent printings of this enormous trade paperback, TPTB print it on lighter paper.
This is, quite seriously, the heaviest 950-page book I have ever read. My forearms grew three sizes from reading it. It's so big and heavy, it was broken up into not two but three volumes for its mass-market paperback edition (for the record, those paperbacks -- with the titles of the three thematic sections of the HC/TPB -- are The Color of Evil, The Medusa in the Shield and A Fabulous Formless Darkness).
However, this anthology is supposed to be a useful all-in-one-volume survey of horror literature from its beginnings to the present day, the present day being the early 1990's, when the book was published. And as a survey, it's a bit of a bollocks on three fronts.
1) Thematic Organization: Generally speaking, I'd say historical surveys need to follow some semblance of chronological order. You can play with this a bit by having lots of sub-categories with, say, three representative stories in chronological order. Having no discernible order, though, and only three vague categories of horror, really doesn't help the hypothetical reader who's new to horror. I can figure out when certain things were published, sort of, and how they link together, because I've read a bloody awful lot of horror. A new reader can't, and thus can't actually get much of an idea of how M.R. James leads to, say, Robert Aickman.
2) Dates: Unless I missed them somewhere, this anthology doesn't provide a clear and consistently deployed explanation of when the stories were published. This is a really irritating omission, and one easily remedied by putting dates on the various stories.
3) ...But He Sure Loves Robert Aickman: Of the 60+ stories in this anthology, three are by Robert Aickman and three are by Stephen King. So 10% of the history of horror is tied up in Stephen King and some British guy no-one who doesn't read a lot of horror has ever heard of.
This wouldn't be as much of a problem if Hartwell hadn't bewilderingly left out Arthur Machen and William Hope Hodgson and limited M.R. James to the short and somewhat second-tier "The Ash-Tree." Machen and Hodgson and James are as central to the development of the horror story in English as Dickens and Thackeray and Joyce are to the development of the English novel. Maybe moreso. But they're only represented by one measly James story.
Now, this anthology deserves some love, not as a useful or even decent survey, but as an occasionally eclectic assortment of horror stories. And unlike the woeful, smug Masterpieces of Horror and the Supernatural survey anthology edited by the woeful and smug Marvin Kaye, The Dark Descent confines itself to stories that can actually inspire horror and terror and unease. So kudos to the unlikely but apt inclusion of Thomas Disch's "The Asian Shore", Gene Wolfe's "Seven American Nights", Philip K. Dick's "A Little Something For Us Tempunauts" and Michael Shea's "The Autopsy", among others.