I tell you what, I didn't expect to come out of it thinking there were things the book did better. The beginning is definitely worse. I liked aspects of the beginning of the book, with a regular guy cracking the code by himself to find a secret hidden in plain sight all along, and him being thrust into fame afterwards. In the movie, it starts with some random unnamed character having already discovered the first challenge and it being common knowledge for a while now. Wade finds a secret in it that is hard to believe that nobody found by accident, and his life doesn't feel super different afterwards. The race is a cool action scene but discovering the D&D dungeon hidden on Ludus felt more personal to Halliday.
Wade's meeting with Sorento was mostly better in the book, there were parts of it that were funny and IOI felt like a major threat because they'd already found out everything there is to know about Wade. In the movie they basically find out who he is by chance, and it's less impactful. The movie fixed the huge problem with this scene though, because Wade actually reacts to the threat of the Stacks getting blown up. He immediately rushes over there and tries to warn his aunt, and isn't sociopathically apathetic to this like he is in the book. IOI in general don't feel like as big a deal, when they had Daito killed and passed off as a suicide it was a real "Oh shit" moment, but that doesn't happen in the film.
Artemis is handled much better in the movie because she's fleshed out more and given more to do. Wade isn't a creep to her, so the romance isn't problematic like it is in the book. The Japanese supporting characters, Daito and Sho, somehow manage to be barely any less problematic than in the book. They don't talk about honour in every second sentence but they're still close to caricatures and are barely present in the movie. Aech is quite a good character, but I don't recall them going into the reason why she presents herself differently within the Oasis which was a missed opportunity.
Halliday's more of a character in the movie, and is done really well. They make the quest for the egg more about his regrets of pushing people out of his life and letting people lose sight of what the Oasis (and creative works in general) was supposed to be. Ogden Morrow and his falling out with Halliday isn't explored in as much detail, which is weird. Kira is just kind of used as a plot device in the second challenge and never mentioned again. Halliday's final scene where he can finally move on knowing that his creation is in the hands of people who appreciate it was quite touching.
The movie makes challenges where players have to recreate movies and games actually work. I hated those parts of the book but they're great here. The second challenge is to find a key hidden within The Shining, which Aech hasn't seen so she unwittingly plays into the events of the movie and it made the audience laugh a lot when they knew what was coming. It ends weird though, because it involves a reference to one of Halliday's games that was never mentioned before this scene so the characters have to explain what the reference is. It's telling, not showing, and comes off awkwardly. The final challenge after the big battle is to play a game on the Atari 2600. Playing the wrong one kills your avatar. The Sixers sacrifice their avatars one by one trying to work out which game is the right one, providing a ticking clock during the final fight as they get closer and closer to the goal. Eventually a Sixer stays alive by playing Adventure. They get to the end of the game, but get killed. Wade realises the point of the challenge - it's not about winning the game, it's about playing and exploring. The challenge is to find the Easter Egg hidden in the game, the first ever Easter Egg hidden in a game. There's a montage of gameplay snippets while Wade gives a speech about how this challenge is meant to remind people of why they play games, and that the Egg rewarded players for engaging with the game and sharing their experiences (which romanticises things a bit because as we all know, Robinett hid his name in the game as self-promotion because Atari was screwing him over, but it works within the context of the film).
One of the weakest parts is the final battle. First, it doesn't detail Halliday's personal connection to the castle (it was a key part of the world he created for he and his friends to play their RPGs in, one of his only social interactions) so it doesn't feel like an important setting. Then all the characters are so desaturated that you can't tell anything apart, they all just look like dull blobs. The Iron Giant being a superweapon is still weird. The Gundam vs. Mechagodzilla fight is much more interesting in the film though, it was surprisingly boring in the book. The ending has the same unearned 'Reality is more real than the Oasis and it's great and worth caring about" message too. The real world of Ready Player One sucks. You grow up in a decrepit RV with your neglective aunt and her abusive boyfriends and corporations try to murder you. In the Oasis you can be best friends with Batman and race a monster truck against a Delorean. The real world is only really great for Wade because he gets the girl and billions of dollars.