Haha! This is a great list, and the planning is really important I think.
Most devs don't wake up in the morning and say "Hey, lets make a new game and just wing it". They plan out with the team and revise and evolve the plot.
I really like making a text file with EVERY little detail and write it in sections with markers where stuff like plot devices are or possible plot devices. Mark any part that needs revising, read and re-read (preferably the entire thing, or anything that relates to that part) until hell freezes over that way you'll know where the plot holes are and can add in more detail. It's also good to write a little bio and summary of the game's character and history so that you can incorporate that into the game adding more (hopefully) depth.
It's also important to believe in your project, it kind of motivates you. If you don't believe in it, why the hell are you wasting you time with it in the first place?