Jo Walton says this book is an attempt to write a society in which the social conventions of the sentimental Victorian novel were instead inescapable laws of nature. It was such a fun experiment, I was kept very happily absorbed and amused by the way the dragon society worked, and by the parallels - I saw them to Trollope and Dickens particularly.
If it was just the fun of the idea itself that probably wouldn't have sustained a whole novel. But there is also an interesting plot and some excellent characterizations. At first the dragons were just oddities but it wasn't far into the novel before I began to care about them for their own sake and wish some of them well and some of them ill indeed.
So four stars for a fun and funny idea and four stars for carrying it out with thoroughness and craftsmanship, not just treating it as a gimmick or a hook.