People are paranoid about sharing their ideas. I was the same way once, thinking that others will steal them or rip them off. It's understandable on the surface, but when you get right down to it, there is no basis for it.
People hijack websites and logo designs everyday (compare Typekit to the new Fonts.com web font service or read about MetaLab's Mozilla fiasco), so why go to such great lengths to keep it Top Secret during development?
Or maybe, you're worried about getting ripped apart, laughed at or criticized. Unfortunately, that's the nature of feedback - some people will be rude and condescending, while others will be encouraging and constructive.
If you keep everything in your head, you might hold copycats at bay (for a time) and avoid a bruised ego, but you also bypass the incredibly useful suggestions and advice that other people have to offer.