Well spoilers ahead:

Every season there were lots of changes I felt had a negative impact on the show.

The one that always comes front to mine was the introduction of the rocks. ~s5 Fillip stealing the Stealth paint as the introduction to the character and the only hint of the plan to throw rocks at Mars was infinitely superior to what happened in the show. Having Ashford as a different character and way more marcos pov time lead to the extra hints and wait.

Book 3 as a whole because of the cancellation was also butchered but they have an obvious reason for that one. Ana and Tilly ( who doesn't die in books) along with holden lost so much here. Particularly the scenes around holdens vision in the ring hub and all of Ana and Tilly's great time together.

S4 as a whole lost so much to cuts in time. This season was huge for miller and holden, both who lost a large amount of time in the light. Elvie also who was 1/4th of the book lost alot and she's arguably more important to the story than almost everyone in the universe.

THE CHURN! They did my boy dirty, after all these years. I can only pray to God they have a movie written they're pitching around. Fucking kickstarter it if you have to.