Interesting, my experience is the opposite. When I have an idea, I try to make mockups or crude specs as soon as a I can, and have some sort of basic prototype within a week or two.

Just like you use feedback from blogs to separate the good ideas from the bad, I usually show a few friends my prototype. I have abandoned several projects at the prototype stage, but that's more a question of "Is this something I want to commit a lot of time to?" rather than "Is this a decent idea?".