This was just a huge episode, there was so much that was great in it, a couple of things that I could quibble with, mostly though it's a feeling of it being quite hard to process and take in everything that happened and the part of the story that's ended.
I was pretty split before the episode about whether they'd deal with the White Walker threat here or not. But when it was clear the army of the dead wasn't being stopped and other way (and I think it was done brilliantly) I knew that they had to kill the Night King by the end of the episode... Unless the second half of the season was genuinely going to be about the Night King winning and finishing off the rest of Westeros with most of the main characters gone. The Night King's walk to meet Bran and the score were brilliant... And Melisandre in this episode was fantastic - it was speculated she might arrive but I genuinely thought she might only show up in the south.
Rather than spreading the action and death of the Long Night through different parts of a season, the show went for containing it within one episode but then pushing the death and horror to the absolute limit within that one episode. I think it'll be a divisive choice, I can already see people online complaining that the White Walkers were defeated so easily after seasons of buildup, but I think that's partly because it's hard for some to realise that this was the climactic battle of the White Walker storyline because it was the first battle of the season. But ultimately I think that even though it was in some sense short (only three episodes since they broke through the Wall), I think that this one episode did deliver what was basically the entirety of the Battle for the Dawn.
Maybe we could have seen or somehow understood (perhaps some sort of vision sequence) something more between Bran and the Night King, but personally I still felt it was a satisfying conclusion. I think the reality that basically everything related to the White Walkers was going to be unspoken and only understood through actions and visuals means that those who want (or think that they want - I believe that people in general are notoriously bad at being able to tell what they actually want out of any story) to get a huge infodump that "explains" the White Walkers better than we already understood then (ideally one that validates their pet fan theory) were going to be sorely disappointed.
The Night King was the big villain of the high fantasy part of this story, a being that was created as a weapon against humanity and life in general, a personification of the supernatural forces of winter and night within the world of ASOIAF. He and the White Walkers were intelligent and strategic, they were certainly not mindless, and some complexity was implied by their actions and the symbolism associated with them, but ultimately though it came from an alien and inhuman mindset, their motivation was utterly hostile to life and light. That makes them simple villains in a way, but honestly while I like the extra depth and complexity implied by the White Walker's actions I was never moved by any theory of them actually having more human motivations or that his was all just some big misunderstanding that could be cleared up. They were a force of darkness straight out of high fantasy, and this episode was the climactic confrontation with that force, and I think it execute the climax of that dark high fantasy story excellently.
Oh, and we still have about 3 hours (or more) to go. In a way it's hard to imagine. There is a fairly simple conflict between Cersei and the survivors of Winterfell still to come but I think the real meat of the season will be the slightly more complex conflicts between the surviving characters of the show, and how they deal with each other now that the apocalyptic threat appears to be over and with the backdrop of a battle for King's Landing.