"Novels that made you work the hardest. Let's assume that you actually finished the book and felt that it was worth the effort" (Goodreads).
Despite the Goodreads instructions, in practice I think there were many different interpretations of 'difficult'. Hence the inclusion of "The Host" by Stephenie Meyer. Anyway IMHO the list could be used as a fairly wide-ranging and challenging TBR.
Criteria for inclusion: at least 3 votes OR 2 votes and 100+ ratings.