I think most people who dislike AFFC have the same opinion: the previous book (A Storm Of Swords) is almost universally considered the best novel in the series, and it ends with a lot of wild cliff hangers. Then people had to wait 5 years for a new book that didn't address most of the cliff hangers, didn't include most of people's favorite characters (Jon, Tyrion, etc), and didn't have a lot of action. It's a contemplative novel. Lots of traveling, thinking, and plotting. And some people hated the new POV chapters.
Personally I loved AFFC on the first read. It's not as good as A Storm Of Swords but I genuinely think it has Martin's best prose.