Anyone with a product to share should want feedback. Doesn't mean that person needs to act on it.
Also, if you are criticized, take a second to think about it before immediately defending yourself.
The worst Show HN threads are where the OP feels the need to respond to every comment on the board to defend his or her project.
Learn to take the criticism.
Positivity is nice, but criticism is what chips away at the block of marble until it becomes a beautiful statue.
No, people saying why it is useless (which is grounded, not gratuitous, negativity at worst) are useful for that reason. People merely saying that it is useless are not useful.
> People just saying great things when they aren't allowed to be negative is kind of meaningless and unhelpful.
A community guideline against gratuitous negativity cannot reasonably be expected to result in "people saying great things when they aren't allowed to be negative" because people are still allowed to be negative, and, in any case, people saying positive things with explanations are just as useful as people saying negative things with explanations.