Strategy Guide
These screenshots were in the official guide, captured on a build close to the Press Kit build:
General

- Life crates ingame don't have Twinsanity's mug on them, they have the classic mug instead.

 - Earning a life animated the icon across the screen, like in the old games.
 - The life counter is far off the right side of the screen.

 - Uka's eyes are misplaced.
 
N. Sanity Island

- The shortcut cave has some sand around it.
 - The purple gem is on the totem instead of the clear one.

 - It also seems to have replaced the blue and red ones above.

 - The camera in Cavern is different.

 - The platform and elevator textures are much darker. Note: The platforms in the drill chase retain their dark texture in the final game.

 - The crate layout at the end of the drill chase is different.
 
Iceberg Lab

- This world checkpoint appears to have no range based activation.

 - After the cutscene, the checkpoint doesn't automatically activate here.

 - This instantly opening door style is different.

 - This checkpoint doesn't exist in the final game.

 - This checkpoint clips through the ground.

 - Rusty's pathing is broken causing him to run into walls and turn around.

 - This checkpoint doesn't exist in the final game.
 
Academy of Evil

- Janitors do not exist in the hub in the final game.

 - The extending bridge looks different, with a different model and texture.

 - Hall Monitors have an unusually huge shadow.

 - Carpets are missing.
 
Twinsanity Island

- Gem tunnels in Rockslide have way more TNT crates.

 - This checkpoint doesn't exist in the final game.