1) Godzilla and Mothra are good, Rodan is neutral, King Ghidorah is bad.
2) Vera Farmiga's character joins forces with an eco terrorist. The other characters won't take her shit when she justifies her actions. One character outright calls her a bitch
3) Vera Farmiga's plan is to awaken the Titans/Kaiju one by one but King Ghidorah awakens all of them, causing mass destruction around the globe
4) There's this device that Vera Farmiga and her ex husband made that sends frequencies to the Kaiju, which are used to control them to a degree. It's a central theme in the movie along with rescuing Eleven from Stranger Things
5) The oxygen destroyer is deployed to try and kill King Ghidorah and Godzilla. Doesn't do shit except make Godzilla flee to heal himself
6) The humans we mainly follow find where Godzilla hides when he's not swimming around and discover ancient ruins that confirm suspicions that the Kaiju were seen as gods in early history with Godzilla king and Mothra queen. Ken Watanabe's character sacrifices himself to "juice" Godzilla up with a nuclear weapon so he can heal faster and take on Ghidorah in the finale.
7) Final scene is in Boston. Mothra dies after injuring Rodan and confronting Ghidorah, while Godzilla is overradiated from the "juicing" and kills Ghidorah with it. Rodan and a couple other Kaiju bow before Godzilla afterward to accept him as their king

King Kong isn't in it but there are constant mentions and references of Skull Island and the Titan there, including that other Kaiju are coming together in Skull Island during the end credits and a cave painting depicting Kong and Godzilla fighting
9) There is a bonus scene at the end with the eco terrorist
10) Not really a spoiler but when King Ghidorah is freed in Antarctica a soldier says "you've gotta be fucking kidding" upon seeing him, which makes me wonder if that's a subtle Thing reference. Most likely is
11) There's a dedication to Harou Nakajima (the original guy in the Godzilla suit) at the end