After reading about 100 pages, my first impression was, "It's like Twilight for grownups, or maybe, The Historian--with vampires!" (Yeah, the latter is a swipe at another book that was well-reviewed by the mainstream media, but left me utter cold. And bored.)
The first comparison came to mind because the romantic relationship shares similarities with Twilight. (FWIW, it probably shares similarities with many paranormal romances featuring a big, bad alpha hero. But Twilight gets the lion's shared of media attention.)
Matthew Clairmont, the blood-sucking love interest, falls into insta-love with Diana Bishop, the witch who can't quite accept that she is a witch. He is much, much older than Diana. He stalks her; breaks into her home and watches her sleep. When Diana returns his affections, he goes through the "No, no, you can't love me; I'm DANGEROUS," routine. He's excessively protective of Diana and often controlling. He's got some kind of hang-up about sex--worst than Edward. Ole Eddy at least got some once he married Bella. (The reason for sexus-delayedous is explained, but it's really...weak.)
I qualify my comparison with "...for grownups" because Matthew, to his credit, hasn't spent his long life attending high school. I mean, if you had an eternity, would you spend it taking high school geometry and American History over and over and over and over....? Matthew has done what you'd expect any sentient immortal to do. He's amassed power, money and knowledge. He speaks many languages and has degrees in all manner of subjects.
Diana, similarly, isn't a virginal teen waiting for her prince to come. She's an adult with a career (tenured history professor). She meets Matthew in an Oxford library, where she is researching the history of alchemy. Though Diana is the very picture of a Mary Sue, she isn't an entirely passive heroine; she's more than a blank placeholder for the reader to imagine themselves with Matthew.
There are three plot threads pulling along the story. First, Diana herself. Her parents were murdered when she was young, presumably by ordinary humans. Consequently, Diana has avoided magic, seeing it as dangerous. Except...she is actually really powerful and the antagonists want that power.
Second, the three magical races in our world--vampires, daemons and witches--are seeking a mythical book that supposedly holds the secrets of their creation. Diana is the only person who can find the book.
Third, The Congregation, an organization that serves as a de facto law enforcement over magical creatures, is violently opposed to Mathtew and Diana's relationship (for various reasons).
Early on, I loved this book and thought it worthy of five-stars. At the midpoint, the number dropped to four. By the last 100 pages it was at three, and occasionally slipping toward two-point-five.
The problem is that it's over-written. Kurt Vonnegut supposedly once said that the secret of writing is to leave out the parts the readers skip. Advice that A Discovery of Witches editor should have taken to heart. The majority of the 579 pages of text are devoted to descriptions of Diana sleeping, eating, dressing, making lovey-dovey faces with Matthew, more sleeping, more eating, more dressing, more sleeping. Seriously, Diana devotes a lot of time to sleeping. Oddly, her actual job--tenured professor who is the keynote speaker at an upcoming conference--is largely forgotten. How's she earning a living? How's she paying her rent? Health insurance? Taxes?
This is where the story starts to stray dangerously into the twee world of Twilight, with a fabulously wealthy sugar daddy, who arrives and takes care of everything. (Except sex. Honestly, what good is he if he won't put out?)
On some levels, the degree of detail lavished on mundane activities reminds me of The Name of the Wind (Patrick Rothfuss), a book that also suffered from its excess of description. The description in The Name of the Wind, however, was at least spent on alchemy and other fantastical elements. You can only read about someone making tea so many times before searching for a spoon to scoop out your eyes. *Glurp*
Unlike, Twilight, the underlying worldbuilding, like its hero, is much smarter. The genetics of daemons, vampires, and witches is especially interesting; some of the historical matters, OTOH, a little less so. (I'm a science geek; so mileage may vary). The plot is driven by a lot more than "Diana is pretty and everyone wants her."
Overall, A Discovery of Witches turned out to be a crazy conglomeration of really good passages, interposed with mediocre, and really, really dull. I'd recommend it to people who love books that lavish thousands of words on...everything.