Eh, there were a lot of decisions that essentially made it impossible to write a good story for the ending. Just off the top of my head:

• Arya using her Faceless Men training once. She could have probably just paid to kill Walder Frey...
• Zero payoff to the big Targaryan reveal - other than it pissing off Dany
• Why Arya decided to go travelling.
• Bran barely used his abilities. Man could've warged into several beasts and fought during the main battle, and outside of being a moody bitch he doesn't really do anything other than act as bait.
• Who was the legendary lord of light? Was it Jon? It kinda seemed like it, but it makes a lot more sense for Arya to be the one.
• Thematically, it would have made a lot more sense for Jaime to reject Cersei, and for this to be the catalyst for her to go crazy - with Dany going crazy from achieving slow victory that chipped away at her claim.
• Varys was too clever for his story to end like it did. IMO he should have been able to escape his capture, and fled away from Westeros.

Unpopular opinion: The show runners did the best job they could, because with the limited time they had there wasn't really an option for a better ending. They really needed at least another two seasons, but the reason GRRM hasn't written another book is because he knows he's written himself into a corner - and rather than try to fix it through TV and give himself an "out" he criticised the ending he probably had a hand in creating, and won't ever finish the story.