I don't know about others, but I am surrounded by quite a few 'idea enthusiasts'. People who believe that an idea is everything, and they get emotionally attached to each of their ideas. It makes it very difficult to critique any of their idea as they will make every effort to defend their idea no matter how weak their argument is. In such cases, I could either tell them 'Interesting idea, talk about it later', when I don't mean to talk about it ever again, but still not hurt the person. Or tell them the truth, that I think the idea is 'stupid' (may be following it up by why I think so if asked, though increasingly difficult with so many ideas floating around).