That they yhrew away all the rules they established before like traveling now took 0 hours and soon. All of this because the last book wasn't written yet so they had to write some story themselves and failed misserably.

A surprising amount went wrong.

While there are a sea of complaints, the biggest for me was that all of the characters stopped having internal logic. Take Jamie, he had a character arc moving from a vain knight avoiding responsibility and having an incestuous relationship with his sister, to having depth, showing that he was wracked with guilt for breaking his oath to help people. Falling in love with a woman for her character and who she was. Being responsible and honorable again. Then the last season came around and he dropped all of his growth to be with his sister.

It's like D&D decided that there would be a cool scene of him dieing with Cersi and didn't care how he got there.

The show reached a point where they'd surpassed where the books were in the narrative and things fell apart. The political intrigue, backstabbing, and subversive nature of the story was done away with in favour of forcing plot points through to get everything wrapped up, with no consideration made on why those characters would act in the way they did.

You could build a museum of horrible decisions and fill it with the last two seasons of Game of Thrones. Whether you watched it or not, the show was a cultural touchstone, and the ending retroactively ruined everything that came before. Many shows have started well and ended poorly, but I'd argue that GoT was on pace to be an all-time top ten series, and there was absolutely nothing good to say about how it ended. Bad writing, bad acting, bad production values, sloppy editing, poor visual design, it was both rushed and too slow, and nothing made sense. If you paid someone to deliberately fuck up everything about the show, they would not have been as effective at it because it would have been obvious.

Everything

Compare it to this... Watching a highly rated chef come up with the most amazing sounding and looking dinner meal over the course of a few hours. You are anxiously awaiting to take a bite and salivating for that moment. When you finally get served your plate and get to that scrumptious first bite, the biggest wave of disappointment hits and you lose your appetite.

I don't know how else to explain it

The best way to summarize it is the writing lol. And they tried to rush plot points way too fast.

The first seasons had the books with everything spelled out.

The end wasn't (still isn't) written. So the show runners were told who gets the iron throne at the end, but not how/why. Also a few other ending points.

So in the show, there's no good reason for what happens at the end. D&D just had shit happen rather than try and show why people would do that shit

Tldr; the whole 6 first season they set up the table for some really juicy stuff and in the last season, some side quests were either ignored or fast-tracked to fit in 1/3 of an episode. Realistically, you had content for at least 3+ more seasons. But since GRR Martin is so slow to write his books (I don't blame him, just pointing out the obvious), the producers of the show had to cut corner and take huge liberties that didn't make any sense

Straight up ruined character arcs