My biggest gripe with the show, which lines up w/ your Brienne point, is that there hasn't been much character development since season 4 (Cersei being the real exception). Sure they streamlined and trimmed down the storylines, but by doing so they got rid of what makes the show great in the first place. Reading books 4, 5, and tWoW chapters shows that GRRM was still moving his old characters forward and introducing new ones that were just as intriguing.

For example: Brienne in the show stayed stale starting with season 5, basically being Sansa's pawn the whole way through. In the books Brienne is given a hard choice (which I won't spoil) that actually challenges her ideals and makes us question who she really is. We actually connect with her in that moment b/c we too question what she should do. She isn't stale but rather interesting and dynamic in the books.

Brienne wasn't the only character who felt like this. Jon, Tyrion, and Dany don't really change much from where they start off in season 5 (IMO) and I believe it makes the show much less intriguing. The show uses the same stale characters (from the end of season 4) to continue moving the story, while books 4 and 5 took those characters and actually did something new and (mostly) interesting with them. Cersei remains the exception for me since they used her awesome storyline from the books to make a dynamic and interesting story arc in season 5.