If I show my web app on HN, and it gains some interest, what's to stop some hot-shot start-ups putting a team of developers onto something similar before mine has the chance to take off?
who are they and where do they live? seriously tho, is this a general paranoia that most developers have and is the answer that there's no point in worrying as there's nothing you can do?
But consider this, no matter what your idea is, there are at least 3 people doing the same thing and closer to launching than you!