Whilst a nice idea, I suspect you might have a hard time growing the community when people can just use Quora for the same thing.
That said, a service (perhaps this exists) where I can give you an idea and pay you a bit of money and in return receive a full market analysis, information on competitors, investments made, upcoming startups working in the area, etc - I feel that would be useful.
Agreed. The value I would see is when I know how the other guys are doing and most importantly, help me decide where I should play.
I believe CBInsights and Mattermark provide a service like that but they are marketed towards VCs and investors as opposed to individual app developers.
That said, a service (perhaps this exists) where I can give you an idea and pay you a bit of money and in return receive a full market analysis, information on competitors, investments made, upcoming startups working in the area, etc - I feel that would be useful.