Character arcs disregarded
Jamie was a bad man on the path to redemption. Never mind that, lets have him revert to his old self at the last minute.
Cersei was the ruthless queen that was on so very many people's kill list. Never mind that, lets kill her by being crushed in a cave-in instead of dying at anyone's hand directly.
Varys was maneuvering around the power structure of politics for his whole life. His every action was an intrigue based on his survival. No ethical barrier stopped him. Never mind that, lets have him turn into a "I only ever wanted the poor to eat!" character.
Arya was the youth consumed by the need for vengeance. We see that resolve soften a bit but one thing was always for sure: She was a hard ass killer. Never mind that, lets have her run around the falling city like some angelic princess, saving people who she wouldn't have given two shits about a minute earlier.
Bron was the rogue who lived for money. This was gradually changed by his relationships with the two Lanister boys. He became so close an ally to Tyrian that he was actually going to face certain death as his champion. They were buddies. Even he and Jamie had an understanding. Never mind that though, lets revert him to a common thug who they barely even know and reduce him back to simply wanting money.
Bran was the Ace-in-the-hole character who was shrouded in mystery and the promise of plot pertinence. What would he bring to the table in the big fight? How will his wharging into animals play a crucial role? Not at all. His character was built up and ended up being worthless. Seriously they could have had him die in S1 and it would have made zero difference.
Kalisi was the Breaker of Chains on her way to liberate the kingdoms from tyranny. She always showed wisdom and reserve when needed. Sure, she got to be annoying at times but the character was consistent. She made tough decisions for the good of the people. Never mind that, lets have her burn the innocent citizens of KL while also putting her own army in danger as they were in the city as well. (should have gone straight to the Red Keep and destroyed only it)
Euron makes you want to punch his face. The promise of his death was sweet. You knew that it was going to be a great pay off watching him die. Never mind that, lets have him die off camera.
Jon is the obvious leader they keep pushing on us and he leads with heart, wisdom, and reluctance. Surely his actions in the final episodes will be glorious. Nope. Nevermind. Lets give all of the important things to be done to everyone else and let Jon run around like a common soldier, repeating the same line over and over.