An excellent concept document that clearly illustrates what the game will be about and how it will look, through fantastic use of external concept art and detailed game treatment text.
The document highlights the need for understanding the overall scope of the game, with some features identified as stretch goals. I recommend going through the process of game modelling as taught in the lectures to help guide your understanding of the complexity of some features. I agree that the key will be ensuring the platformer mechanics are tight right from the start.