And then you have the Boltons, who everyone in the North absolutely despises in the books, managing to easily summon an army to crush Stannis without any difficulty whatsoever. In the books, the North despises the Bolton-Frey alliance and they're barely maintaining being on top. We get some really stirring moments where through Davos and Theon we get to see how The North Remembers and how despite all of Ned and Robb's failures, their vassals are still fiercely loyal and want to fight for them even in death. The Winterfell that Stannis is marching to is a kettle about to boil over from the tensions inside, especially with Freys being murdered thanks to a certain mischief caused by a character that isn't quite as dead as he is in the show.
I had this question as well when I watched that scene—"how on earth do they have that big of an army?!" When your vassals don't like you, they don't give many levies to your army. Lessons from Crusader Kings II. I am definitely disappointed that Winterfell wasn't taken back in the finale.
Then you have the Jon storyline, with the "For The Watch" storyline. In the book, Jon forsakes his vows and goes to stir the wildlings to go march on the wildlings to go save Arya. Just as he's gathering to march away, Bowen Marsh with tears in his eyes stabs Jon Snow because he legitimately believes that Jon Snow is endangering them all by letting a wildling army run loose in the realm. In the show, this is boiled down to Alliser Thorne being kind of an asshole and decided to murder Jon with a gang of bullies that don't like him very much, despite Jon doing none of the questionable decisions as a leader that otherwise led him to this path. Instead of it being "For The Watch", it's "For Me And My Buddies Because Fuck You".