Great post. I've been guilty of this at times and will try hard to stop. It's hard to strike a balance on knowing when a person just needs to try and idea and when they should really reconsider it entirely. Also, I would have told the founders of Twitter that I think their idea is stupid and that no one is going to post 140 characters about their breakfast. I would have been wrong to be discouraging. I guess the key is "constructive" feedback, this no matter what your overall opinion of the idea can help point them in the right direction.