Lay people are easy to convert, you can simply answer questions intelligently. Someone with no interest or experience in the area isn't very emotionally invested either way, and can be convinced with ease.
On the other hand, those with real knowledge or experience are also easy to sway, but once you show them a glimmer of potential, credibility is on the line. The inevitable response to a good idea is a quick reactionary criticism. "Yeah that's a good idea, but it would be much better if you did X". It seems like a kind of flexing, or posturing to one-up your concept. I've encountered this several times now.
My reaction is to exhaustively answer questions and politely offer answers. This tactic seems to be met with defensive resentment, and it really sucks the excitement out of the conversation.
This is a real communication problem for me, I'd appreciate any advice.