Very interesting. Seems to work pretty well so far. A few comments.
1. I have more than 100 shelves in Goodreads, but the plugin only retrieves the first 100. I suspect this is a Goodreads API issue as opposed to an issue with your plugin, but it's something to look into (at this point I don't foresee personally trying to access any past the first few anyway for my own purposes, but it's something others may need to do).
2. Currently one can add a set of books to a Goodreads shelf, or pull all of the books from a shelf and add them to Calibre (and/or add tags or similar options to the data in Calibre. Once a book is linked, I'd like to see more book<->book operations (as opposed to shelf operations). For example, syncing my rating of a book in calibre with the book in Goodreads (or visa-versa). Or creating tags in calibre for a book that match all of the shelves it's on in Goodreads (that is, rather than pulling all books from a shelf, pull all shelves for a book).
Anyway, these are more long-term wishlist (and API allowing) issues. Still a very nice plugin as is.