I think the character of Cersei was severely short-shrifted.

Early in the series she coins the term: "Game of Thrones" and she gets many memorable scenes and quotes (I especially liked her rejoinder to Littlefinger's attempt at bragging: "Knowledge is Power").

She is the architect of a lot of the decision making out of King's Landing, until she runs afoul of the encroaching religious establishment.

She rightfully owned the moral complexities and ambiguities of the series, and deserved a bigger say in the development, which was denied her. In order to bring the tendrils and threads together to tie a string round the bouquet at the end she was turned into a simplistic baddie. And she had an ignominious end as a minor character that she did not deserve.