I finally got around to seeing GxK this week. It's very silly but I enjoyed it. My wife and I left the theater kind of giddy, like a couple of kids high on way too much sugar. Now I had seen a rough cut of the movie last year on the Warner Bros. lot and noticed a few big changes in the final theatrical version.
Spoiler
There was a bit more emphasis at the very beginning on Monarch trying to assuage the fears of various governments regarding Godzilla. There's a lack of trust in Monarch's ability to keep the public safe due to the collateral damage from Godzilla's various battles. Rebecca Hall's character does mention that the big guy is protecting the world from rogue titans and preventing further destruction, which is a line kept in the final film.
The battle between Godzilla and Tiamat was added to the movie. Originally, he absorbed all of the radiation from the power plant in France and went immediately into the ocean. He was tracked via submarine way out into the ice where they discovered his husk floating in the depths. Godzilla had shed his entire skin, and then we see the evolved version of him rise up from the waters.
Mothra was never originally in the film. There was another Titan known as Fosfera that was introduced at the end. It was a white winged dragon with slightly insectoid features but it basically plays out exactly as it does in the final film. I much prefer Mothra if I'm being honest. I also noticed that every time Mothra is mentioned in the movie the dialogue is said offscreen. Clever editing done to substitute the creatures.
Overall, I had such a blast with this movie. I hope they continue with the Monsterverse going forward.