My take is a bit different. In gaming, if you listen to and believe the experts, who have a gaming background and how consult with the major studios, the most critical, and hardest piece, is the monetization model.
If you are using any revenue model, except ad supported, ie, F2P/subscription, this is the one area that is the most difficult, most challenging, and most competitive. Only 4% of video games are successful, a much lower rate than almost any category of product/service that I know.
So even from an investor viewpoint, I would want to know:
a) how you are going to beat the odds, ie, materially decreasing the chance of failure at 96%.
b) how is your game sufficiently positioned as to be differentiated?
c) who is your player...and I mean with some degree of specificity so that you can discuss reach and distribution of your game, the lack of which is another real cause of failure.
d) do you have any proof of the above? Could be as simple as white boarded visuals/flash mockup shown to prospective players (see c)) with their comments consolidated and presented to show potential.
Add'ly, and maybe you know already and this is redundant, I would strongly start with a clear, well thought out, monetization model, then design the compulsion loop around it, then design the "game" aspect around that. A previous client took the opposite route - the lipstick (art) looked fantastic, but the pig underneath..not so much.
My two cents.