Not sure if it's too meta to comment on whether true optimism can draw on constructive negativity as one of its mechanisms for self reinforcement, but here goes...
Sometimes feedback that points out problems is misinterpreted.
I find that when presented with a new idea (either from outside, or it could be my own brain coming up with it) my first inclination is to ask: OK, what obstacles do we have to tackle to make this real? Those obstacles represent a todo list of steps. Looked at one way, they can seem negative. Looked at another way, they can seem like the fundamental tools of a true optimist.
(What do I mean by "true" optimist? I mean someone who views possibilities in a pragmatic way and wants to figure out how things can happen despite obstacles, rather than just hoping the problems will vanish.)
On HN, the new ideas (and recycled ones) flow fast and furious. As does the negative feedback. And also some gratuitous negativity, unfortunately, as you point out.
I hope you find what you are looking for. In the meantime I find HN to still be pretty decent.