It's just other peoples' ideas that are overrated. Mine is pure gold.

Partly paranoia, and partly because overrated doesn't mean worthless. I occasionally advise startups we've funded to keep some part of their plans secret. (You never want to keep everything secret, or it just advertises you're keeping a secret.)

Fortunately, YC folks have you to advise them. I often find myself in the predicament where I need to discuss an idea/ get some feedback / advice from someone who's familiar with the startup scene. Friends - family are too out of touch with tech startup world to provide any relevant,useful input.

I'd say YC should start like a monthly session (paid) where a limited set of entrepreneurs can get advice/input? Maybe far stretched considering your time constraints, but just thinking out loud.

Guy Kawasaki said it best:

Nobody wants your crappy idea, everyone is already working on their own crappy idea.

Yeah, I think a lot of it are delusions of grandeur and fear that people will mock the ideas. Note--stackoverflow.com was very open all along, very successful, and as far as I know virtually no competition.

I like the concept of ideas and execution being multipliers, put forth by 37signals (and surely others).

http://gettingreal.37signals.com/ch06_Done.php

Of course, the reality is non-linear, and the exact shape of the idea/execution/expected-profit manifold is quite up for debate.

Very simple: everyone thinks their idea is the best thing since sliced bread. So they won't tell anyone, by fear of being copied.

I have a scoop for you: people couldn't care less about copying your idea. However, once you are successful, everyone will want to copy you.