This book seems to generate more diverse opinions than most Martha Wells books, and for good reason: to the extent that there is a pattern to Ms. Wells' books, this one diverges most from that pattern.
It starts with the setting. Most of Ms. Wells' other books are set in locales that while original are easily comprehensible. This is due to her use of historical archetypes as inspirations for her invented settings: 19th century England and France for the Ile-Rien books; Southeast Asia for "Wheel of the Infinite." "City of Bones" on the other hand is a wholly original setting, a post-holocaust city rising out of the desert with its own history, class structure, racial prejudices, and magic system. There's thus a bit more description, a bit more explanatory exposition here than in Wells' other books. Those less interested in world-building as a source of wonder may find it slow, but I loved this aspect of it.
The nature of the setting ties neatly with the plot, an archaeological mystery that gradually reveals some of the past history of the land even as it takes the characters from slums to palaces, desert ruins to universities. One aspect I really enjoyed about this book is that it isn't rushed -- some of the more recent books Wells has written ("Wheel of the Infinite" and "Gate of Gods" come to mind) have felt like too much new material was introduced in the last 50 pages. "City of Bones" really builds the story so that the end, while impressive, feels like a logical, understandable outcome of all that came before, and it gets the attention (and page count) it deserves.
The hero, Khat, is an Indiana Jones-type: capable as an adventurer but someone who'd rather be studying the mysteries of the Ancients. While not amoral, he's roped into the story not by any great need to do good, but by a desire to learn and (as a racial minority relegated to the slums) to simply earn enough money to survive. The heroine, Elen, is perhaps a bit less capable than the women in other Wells books, relying a great deal on Khat for assistance early on. Much of her weakness however is psychological, and part of the enjoyment of the book is watching Elen grow into her abilities. There is an element of romance in the book, but less so than in most of Wells' other books, and it's handled differently here -- there's more a focus on the things that can keep people apart than the ways they can be brought together. Both characters grow throughout the book, but both end -- in a good way -- as still far from finished products: this is a fantasy that "feels" very realistic and true.
That trait carries through to the villains of the book. There is ultimately a source of opposition, but not all characters that look fair are, not all characters that feel foul are, and those that are foul have believable, thoughtful reasons for being so. This is not a grim or gritty book, but politics and shades of gray do figure just as strongly here as Saving the World from True Evil.
Overall "City of Bones" is a thoroughly enjoyable book, one I'd recommend to anyone interested in reading something that while "light" manages to push the bounds of genre fantasy. I'd especially recommend it to those who have read other Martha Wells books, as this one really illustrates the breadth of imagination that she's capable of.