Good art is not merely entertaining. It's didactic, it broadens the mind, it makes you think. Similarly, a good ending is a resolution bringing a fresh perspective to a problem.
GoT's finale was incredibly subversive. Not a happy ending, but full of valuable lessons, which is precisely why it was a great ending.
The last two seasons received a lot of hate, most of the time for the wrong reasons. No, the plot was good. Yes, the pace was too fast.
GoT is not merely about dragons and zombies, it's a political essay trying to answer a complex question: what is a great ruler?
The series was a huge political campaign where each spectator wanted his/her own favorite to win. People who didn't see their own fantasy come to life in the last episode ended up frustrated and blamed the "poor writing". I can't disagree more, I believe the series does a great job at mimicking GRR Martin's witty, highly-politicized, writing style.
Tyrion explained really well why Daenerys wasn't worthy of ruling during his meeting with John in his cell: the mother of dragons was, and always has been, a tyrant. Daenerys' twist is brilliant because it explains how people can be led to serve and praise tyrants: with emotions, Manichaeism and good storytelling. Daenerys only dealt in absolute - yes, like a Sith Lord - everything is either black or white to her. We were lead to believe all the murdering she committed were done rightfully so, but the truth is more complex than that and her underlying nature finally revealed itself in the fifth episode of the last season: Daenerys' purpose is to dominate, as illustrated by her spirit animal. Dragons symbolize wealth and strength, but also greed and egotism. Daenerys didn't hesitate to twist her own reality with a good story to fit her dreams of grandeur. She always lived in a dream, she died with her illusions.
Daenerys' blindness became apparent during her last encounter with Jon Snow when she told him of her motives and how he should better than anyone else, after all the events he went through. This line also illustrated his character perfectly: Jon is bound to know nothing, highlighted by two references to this famous line during the last episode. Ultimately, his story ends as it began, his cycle repeats itself, he will always repeat the same mistakes, learning nothing. Jon Snow is a hero, a knight ideal, he is meant to serve and protect others. Jon Snow is a natural leader because his virtue inspires others. He doesn't want to rule because he acts out of necessity, out of emotions. He lacks the calculative mindset most politics display. According to Machiavelism, Jon Snow is not fit to rule, his heart is too pure, which is why he ends up hanging out with the wildlings: Jon Snow's happiest time was with Ygritte - he is a simple man, so he joins a nomadic tribe of ingenuous "free folks". When you see it this way, he had a good ending.
Bran becomes king, why is that?
GRR Martin depicts machiavellian characters as the aptest to rule, but Bran has something even more valuable than a mindset prone to politics: he has ultimate wisdom. His powers make him omniscient and the closest to truth/reality, the way things are meant to be. Bran is thus an allegory of the philosopher king, the ideal ruler "who possesses both a love of knowledge, as well as intelligence, reliability, and a willingness to live a simple life".
Tyrion is not only wise, he constantly learns from his mistakes, which is why he is fit to become a great advisor: he is not afraid to do his best and fail, because he will always find a way to get back on his feet. All the people ruling Westeros are now the wisest of the wises, including Bronn who was the ultimate cynic and among the most pragmatic characters of the show, and with the exception of Sansa who became the new allegory of Machiavelism after learning everything from Baelish.
The only problem with this season was its pace, which made things hard to understand. And yet, the message is still there, which is what matters most in a good show.