Is it worth posting to say 'I have an idea' or 'I need someone to program for me'?
See the 4th question
What makes a demo worth posting?
Any demo is worth posting, but make rules that forbid people to post various threads about different versions of the same game. Let them just have on thread about their game, and keep everything related to that game in that thread.
How can you tell if a game is trustworthy?
Any game is trustworthy. As long as the one developing it is focussed. As soon as they lose interest it goes the wrong way.
I'm thinking about making a specific thread for all 'I have an idea' posts; is this a good idea?
It is a good idea but make it more like into a collaboration thread. Where people can request help on a project but can also offer help to others. The best Idea for this would be to assign someone with the duty of keeping the first post up to date with a list of persons searching for and offering help.
Hope this helps you with the decision. I think it is a great idea to split the fan corner up in maybe two section but not more. Also a set of new rules specifically for the fan corner will help. Good luck with this
