That sucked, but what really got me was the way they resolved Tyrion's arc.

In the books, he's constantly revisiting the trauma of his relationship with Tysha. She was the first woman Tyrion was ever with, and he wound up falling for her and marrying her. Tywin found out, and ordered her to be raped by his entire guard, then ordered Tyrion to rape her last before he sent her away. Tywin reveals he did this because she was just a whore who was conning Tyrion, and Jaime admits that he knew about it. This obviously fucks Tyrion up, both because of the obvious horror of what happened to her and the implication that nobody will ever love him genuinely. The whole Shae situation reopens this wound.

When Jaime and Varys come to smuggle Tyrion out of the city, the subject of Tysha comes up. Jaime admits that he lied, and that Tysha was actually just a commoner who fell for Tyrion. Tywin forced Jaime to pretend otherwise to teach Tyrion a lesson. In a rage, Tyrion admits (falsely) to assassinating Joffrey, and tells Jaime (truthfully) that Cersei is fucking just about everybody she can behind Jaime's back. He then sneaks into the Tower of the Hand, murder's Shae in cold blood, then has an argument with Tywin about Tysha. This culminates with him shooting Tywin the second he calls Tysha a whore.

In the show, all the Tysha stuff is dropped. Tyrion sneaks into the Tower of the Hand unprompted, and tearfully kills Shae in self-defense. He then kills Tywin for calling Shae a whore. He also has a tearful, sentimental farewell with Jaime. All in all, it was a much more sanitized happy ending for Tyrion, and wound up robbing both him and Jaime of their purposes in future seasons. Jaime was supposed to grow distant from Cersei, instead he keeps doing her bidding and we get whatever the fuck that Dorne arc was. Tyrion was supposed to stay on his trajectory of becoming a vengeful monster who wants nothing more than to hurt everybody who's wronged him. Instead he becomes a cheerful drunk who likes to make quips and ask invading queens to pretty-please not kill any of the people who wronged him.