First and foremost, this site is all about Legends
fan games. A demo that contains non-Legends graphics with the intention of replacing them later with Legends graphics is not ready for Sky Pirate Arcade.
Give the players a glimpse of the bigger game to come. This can be as simple as a one room game that shows off the lead character's motion engine, or as complex as a full level of play complete with enemies. A demo that is nothing but an intro (or similar cinematic, non-interactive element) is an example of a demo that fails to represent the finished project, and is not ready for Sky Pirate Arcade.
Fix all major issues. If programming from scratch this may be difficult as some bugs and system incompatibilities may not be apparent on your system, but in a program like Game Maker or Unity it's pretty inexcusable. A demo that freezes, crashes without warning, or causes other problems for the player's computer is not ready for Sky Pirate Arcade.