this is like asking a fisherman where his best fishing spots are
there are a bajillion worthwhile business ideas... they're a dime a dozen (and that's paying too much). The problem is IMPLEMENTING them, which essentially no-one ever bothers too.
way more folks who hear you idea will say "shit, i wanna use that" than "i'm going to spend a year of my life making an app to compete with that, and then another year marketing it and...."
Can you list a few of the best ones for me?
It absolutely happens.
The benefits of sharing an idea and getting feedback far far far outweigh the minuscule risk that sharing an idea will spark a competitor that will end up outcompeting me.
Now - having said that - posting your ideas in detail on a public thread on Hacker News doesn't necessarily have the same benefits, so I can see the hesitation ;)
In my experience it really depends on the type of idea, the other types of validation you already have, and your existing relationship to the funding network.
The more moonshot the idea, the more validation you can get outside of funding pitches, and the less cozy your relationship with the (well-networked) money, the more risk you get from sharing.
Many of these companies will pitch the big-picture what and why but not disclose the how, which is the magic. On the other hand, if the what and why are all the value and you share them before you’ve at least somewhat captured them then you risk giving away the opportunity, because the how is just what you’re asking for: the resources.