I don't usually follow the TV threads (don't have time) but had to have a peek at this one to see what everyone thought!
Some of my random thoughts on it:
I guessed in episode 2 that Arya would kill the NK and told DH so! So was very smug when it happened. In fact I thought there were some quite big hints, especially about Gendry making her special weapon - though it was a surprise that she used the dagger in the end (I thought she would throw the spear, like when the NK took out the dragon!).
I was quite chuffed it was her though - I don't think it would have been right for it to be Jon. If it was him the only way he would have done it would be run-up-and-hit-him-with-sword, but we know the NK isn't that easily killed by normal methods (as Theon, the fall from the dragon and dragon flame showed). Jon may be a good fighter but doesn't have the sort of supernatural talents needed (OK he came back to life but that wasn't really his doing!), whereas Arya's training allowed her to get in close and hit just the right spot before he could kill her.
As for Theon's charge, I read this as him being close to dead anyway after having dispatched all the other attackers (the look in his eyes just before!) and wanting to finish well; and what Bran said to him was like a goodbye, which indicated Bran also knew he was about to die one way or another (and perhaps also that Bran no longer needed him to stand guard). So it made sense to me.
I thought Sam did pretty well actually, he seemed to be holding his own in the fighting most times we got a glimpse of him I thought. If he had been a total liability I'm sure he would have been dead pretty fast!
I too found it quite improbable how many of the main characters survived, especially those in the thick of the fighting in the castle (Jaime, Brienne, Tormund etc) - with so many wights and then their own dead being revived against them it didn't seem as though anyone in there should survive. But I liked the thought that it's flipping our (now)-expectation of unexpected main-character deaths by giving us unexpected survivals!
As for the suddenness of the NK's death, wasn't that how it always had to be? As long as he was alive, it would be ultimately impossible for the living to win, as every one who died could be raised up against them again - so not only the survivors would be shrinking all the time, but the attackers constantly increasing. So literally the only way for anyone to survive would be to take him out. Of course he could have stayed back in safety instead of coming for Bran, but I think that would have been the end of the series then
As for why it needed to be in episode 3 though, hopefully we'll find out soon!
Oh last thing for now, I was v disappointed in Sansa in the crypt - I thought she was meant to be becoming a leader and Lady of Winterfell (and some people speculating whether she'll even end up as queen), so I was expecting her maybe to have some words to keep up the courage of the other women and children, and/or be ready to have a try with her dagger when the corpses started emerging, or something at any rate - but she barely said/did anything and seemed to ignore almost everyone else (except talking to Tyrion - though I did like that scene), and when she did speak it was putting people's backs up (Missandei). I hoped for more from her!