In short, Six of Crows follows six thieves tasked with an impossible heist. Not only must these six (who don't trust one another) work together, they need to best an incredible array of forces lined up against them.
The word that keeps popping into my head when I think about this story is "sizzle." While it does have a few action-heavy set pieces, most notably near the conclusion of the book, the main source of tension in this book is between our POV characters. They all have their own secrets and motivations, and as loyalties constantly shift, there's no telling what will happen.
While the worldbuilding and the action are superb when they appear, I don't think fantasy fans looking for incredible fight scenes will be excited by this story. That "sizzle" I keep thinking about is between the characters, and while it builds and builds, it only explodes at the end.
But I loved this story. The characters are masterfully written, and Bardugo has a gift for revealing character through small moments. I read this one slowly, savoring the descriptions and shifts in loyalty as more information was unveiled.
Needless to say, I'm excited to begin the second half of the duology, Crooked Kingdom, next.