This is the first time that Nikolaj Coster-Waldau receives top billing. However, he is only seen in passing and has no speaking lines.
The episode title is a reference to the naming standard used for kings of Westeros; the newly crowned King Tommen is now known formally as: King Tommen of the House Baratheon, First of His Name, King of the Andals and the First Men and Lord of the Seven Kingdoms (basically meaning "King Tommen The First"). The coronation was only indirectly mentioned in the books.
Arya and the Hound discuss Syrio Forel, with Arya believing that he was killed by Meryn Trant in The Pointy End (2011) because he only had a wooden sword, and the Hound claiming that if Forel had a decent sword, he could have easily defeated a mediocre swordsman like Trant. Forel's fate was not shown; he could have conceivably picked up a sword from one of the other disarmed Kingsguard members, but when Maisie Williams asked the showrunners if Forel could have survived, they assured her that he was dead.
In the novels, when Sansa and Littlefinger arrive at the Eyrie, Lysa also wishes to marry him immediately, and privately reveals that she was once pregnant with Littlefinger's child but her father forced her to have it aborted. In contrast to the show, Lysa doesn't realize who Sansa is until the next day, and is unhappy about her presence. The wedding is described from Sansa's POV: she needs to help carrying and undressing Littlefinger for the bedding ceremony as Lysa only has three ladies present. Even as he is marched into Lysa's chamber in the nude, he keeps smiling at Sansa. Later that night, a drunk singer called Marillion tries to sexually assault her, but another knight saves her. Lysa also blurts out the truth about killing her husband and sending the false letter to Catelyn, but much later and in the presence of Sansa and Marillion; Littlefinger feels uncomfortable about the revelations and tries to shush her, but in vain.
With the revelation that Littlefinger and Lysa conspired to poison Jon Arryn and shift the suspicion to the Lannisters, they have effectively started the chain of events that led to the War of the Five Kings: Jon's death and the possible role of the Lannisters in it made Ned investigate these affairs as Hand of the King, which caused him to proclaim Joffrey as an illegitimate king. Littlefinger can be considered the true mastermind as he convinced Lysa to do these things and also claimed that the Lannisters tried to murder Bran with a dagger belonging to Tyrion. Tyrion's subsequent arrest by Catelyn kicked off the first hostilities in the conflict between Starks and Lannisters, which culminated in a full war after Ned's execution. Ironically, Littlefinger's machinations also unintentionally ended in the death of his beloved Catelyn.