In an interview, RobTop, the creator of Geometry Dash, said that the well-known level "Ultimate Destruction" was a test, so it was never planned to be released.
The level ratings are based on the Newgrounds emoticons.
The reason Ultimate Destruction was never planned to be released was because RobTop never got the rights to use the song.
When you beat Hexagon Force in Normal Mode, you get an icon that looks like a Minecraft creeper.
A lot of the icons are references to video games, movies, etc.