I just finished watching S7. Watched S5-7 in short succession, and I've noticed a pattern in the show's writing that started as early as S5.
To be clear, I have no problem with the show deviating from the books. Especially given that books 6 and 7 haven't been published yet, they really had no choice, and even if they did, some deviation would be necessary, unless they wanted to make each season 100 episodes long. I even like some of the changes. Tyrion meeting Dany early on, for example.
But I think most people agree, the show has taken a notable downturn since S4. I first noticed it when Littlefinger gave Sansa to the Boltons.
The reason... the established reason... that Littlefinger arranged the whole Purple Wedding thing, was because he wanted Sansa. He wanted her for himself. Not only that, but Sansa really had to stay in hiding because Cersei had a "warrant" out for her. The only reason Littlefinger gave Sansa to the Boltons in S5 is because the writers wanted him to -- never mind that it makes no sense with his established motivations -- and the only reason both Sansa and Littlefinger survived S5 is because the writers apparently never considered how Cersei would react.
To me, this is the root of the problem. You can hear it in the interviews with them. They constantly talk about what they want to have happen. They keep trying to give the fans the things that they want to have happen. But it's all for nothing if you forget the basic rule: what should happen, is what would happen, given the characters involved.
Littlefinger should not have given Sansa to the Boltons, not because we don't want to see that, and not because that's not what happened in the books, but because that's not what Littlefinger would have done. And Cersei shouldn't have allowed it, because that's not what Cersei would have done.
ASOIAF has great worldbuilding, it's true. Many fantasy series have great worldbuilding. What makes the series great is the way everything... everything... that happens, in the entire series, feels organic. It feels natural. It feels like everything that happens is a direct result of the characters... all 1,995 diverse characters... acting like they would act in that situation. That's what makes the world feel real. That's what makes us care about the characters. That's what makes the story great.
It's inconvenient to write that way, sure. Sometimes, it means you have to give up on the cool things you want to see happen. But it ultimately results in a much better story.
S6 and S7 can basically be summed up as, "we want to see this happen, and the fans want to see this happen, so let's make it happen, and that'll make the fans happy." But writing that way is like giving us nothing but ice cream to eat. Sure, it's tasty for a while, but there's no substance there. Without organic character behavior, nothing else works.
Some other specific complaints, off the top of my head:
Not exactly a character complaint, but there was a change from the books that I didn't care for, just because I didn't care for it. I liked the meaning of the words, "the North remembers." How the entire North banded together against those who betrayed the Starks. While I love Arya, I felt having her responsible for everything cheapened things.
Speaking of Arya, the House of Black and White is rather dedicated to its secrets and its religion, and I don't think it would let her go so easily. Even if she wanted to go. Which she wouldn't. At least, not just on the basis of being asked to fulfill a contract.
Also speaking of Arya... what the hell was up with her behavior towards Sansa in S7? Seriously. What the hell?
What exactly were all of those knights and lords showing up to Arya's/Littlefinger's trial thinking? Did they know in advance what was planned? If so, how is that smart? And if not, why did they just go along with the change of plans?
Speaking of Littlefinger's trial: how does it make sense that Littlefinger was behind the cutthroat attack on Bran? That makes no sense. Littlefinger was a) all the way in King's Landing at the time and probably didn't even know that the younger son of Ned Stark had an "accident," and b ) in love with Cat, Bran's mother. Contrast with the explanation in the books, when Cat asked Littlefinger about the dagger, Littlefinger presumably recognized it immediately as belonging to Joffrey (who did it because he heard Robert saying that it would be better for the boy to die, so he did it to make Robert proud), but Littlefinger couldn't very well tell Cat that it belonged to Joffrey, so he made up a story about losing it to Tyrion.
Not exactly a character complaint, just something I didn't care for: Danaerys was much more cunning in the show than in the books. In the books, all of us readers could see the machinations going on in Meereen, but she couldn't, when Hizdahr zo Loraq kept asking her to marry him and promised to make the attacks by the Sons of the Harpy stop if she married him. It kind of made sense that she would be naive, given her lack of experience with court machinations, and it emphasized very nicely just how much she needs somebody like Tyrion. I know the writers wanted her to be a strong female character, but... she is strong. Strong and flawless are two very different things. If they'd stayed true to the books in this, it would have shown a strong character who still has room for character growth.
Back to character complaints: Why did Tyrion believe that they could convince Cersei to fight alongside them, just by showing her a wight? Why did he ever think that she would see it as anything other than a tool to use for her own ends? That's all Cersei, as portrayed in the show, is capable of seeing. That's always been an established fact.
After they saw the army of the dead at Hardhome, why did they think that a small band of people would be enough to abduct a wight?
Why would Elia consent to an annulment? For that matter, why would the High Septon, after their marriage was already consummated and she had given Rhaegar two children? And on a related note, why would Lyanna name her child Aegon, when Aegon is also the name of one of Rhaegar's other children? All of this screams "convenience" and "fan service."
Not a character complaint, just a practical one: Sam got from Oldtown to Winterfell awfully quickly. In his horse-drawn wagon. With a woman and a toddler. In winter. It took him, what, a week?
So... yeah. Those are my thoughts on where the show went wrong. In all fiction, but especially in fantasy, it's always best to start with what the characters would do. Not with what you want them to do. GRRM does this expertly. The show's writers, OTOH, are looking at it backwards.