It's important to start with what this volume is not. It's not a collection of a particular type of horror story; Datlow's taste, while tending toward the subtle over the blatant, is wide-ranging, and includes stories traditional and modern (to the extent that these labels are useful), long and short, serious and comic. Some are closer to dark fantasy than "horror" as some readers narrowly define it. This book is also not necessarily cued to your specific tastes. Datlow has not magically reached into your head and selected nineteen stories and two poems that you are guaranteed to love. Cover copy notwithstanding, Ellen Datlow does not know what scares you personally. To say that a book is "not for everyone" is often a form of back-handed criticism, but here it's just a fact.
With that out of the way, I can say what this book is: a collection of fine stories displaying the scope of the modern horror story. I can't say that I unreservedly admired all of the stories here, but I respected each one's craft. A new anthology edited by Datlow is a guaranteed purchase for me, and the reason I keep coming back is that I never find a story whose appeal utterly baffles me. Sometimes I don't find them as successful as they might be, but I never think "What the heck was ~that~ doing in this book?"
I'll highlight a few stories I particularly admired. Margaret Ronald's "When the Gentlemen Go By" is a brief, chilling story about a small town and the price it pays for its happiness. Again, traditional-sounding stuff, but the story's structure allows it to build to maximum effect, and there are a number of chilling moments along the way. It's also an interesting contrast with "The Hodag," a very different but equally effective small-town horror story elsewhere in the volume. "The Rising River," by Daniel Kaysen, is a sharply-styled, twisty little story about a girl who can talk to ghosts, or can she? Graham Edwards' "Girl in Pieces" is a mystery/science fiction/fantasy/horror hybrid. It's also a comedy. It sounds too busy to work, but in fact the noir-derived prose style makes it all fit together nicely.
In addition to the stories and poems, the volume also includes Datlow's summation of the year in horror publishing, an eminently useful list of novels, collections, anthologies, magazines, and other outlets for horror prose. With a genre that's so dependent on small presses, this essay is a much-needed annual resource for finding works you may have missed.
This is the kind of book you might want to look over before buying if you're not familiar with the editor's taste. Horror is (and should be) a broad church, so it's worth looking at some of the stories, and the editor's recommendations of other books in the summation, to get a sense of whether it's right for you. If it is, you're in for some excellent tales.