Character, character, character. Every single personality in this book WAS a personality, good or bad, and popped right out of the page. This book reminded me a great deal of Pride & Prejudice -- there were similarities everywhere, from the reasons they initially disliked each other to how the heroine had nothing much to recommend her except for a pair of speaking gray eyes. Sylvester is like that most famous book if Jane Austen had penned rollicking moments of crazy daring, and was a little less reserved in making fun of her society caricatures. Nugent Fotherby certainly wasn't intended to please the audience, but the fact that he was by far the character I enjoyed reading most speaks scads about Heyer's talent.
I think every book could benefit from the addition of a Nugent Fotherby character.