Earlier quoted context omitted.
What would you prefer? Rolling applause for every idea anyone presents? One benefit of criticism is we might get to read an angle of attack we hadn't thought of and thereby improve our ideas and designs. Sure, when people blow off an idea by saying "pointless, let's save the baby dolphins first" I don't find those comments helpful, but HN almost always has far more in-depth critiques. Edit: fixed a word
And the negative comments help wipe out a great bit of future competition. If you kill an idea before it gets started, you reduce the risk of it actually working. Here to the crazy ones.
I encourage people to say, "This won't work, here's why, but here's how it could."