I don't get the hate for the ending. I think a lot of the criticisms missed the point. Everyone wanted a person who was consistently going mad over the course of the story to take the throne.
I think the disconnect here is that first few books/season of A Song of Ice and Fire set up the reader/viewer to expect a downer ending. They are engaging specifically because they break conventions by killing off so many of the protagonists.
In a traditional story a happy ending is satisfying because it fulfils expectations, but in A Song of Ice and Fire the 'happy' ending manages to leave the viewer hanging because it breaks the implicit promise of the first few seasons. I fully expected (and wanted) the series to end with the last human dying as the dragons face off against the White Walkers.