The flaws (or potential flaws) in a given system are almost always more interesting to discuss than the ways in which it has succeeded. Praises ring soft, criticism is fun to debate.
I agree that it can be interesting and useful to discuss flaws, but potential solutions are what make the discussion worthwhile. Saying "This will never work for X, Y, and Z reasons" creates a much less interesting debate than "This has flaw X, but I think it could be solved by this change. What do you think?"
It frequently appears that people react negatively on HN purely based on instinct, without doing any research. The response to Dropbox's launch is particularly illuminating:
https://news.ycombinator.com/item?id=8863 (I think that every Dropbox user can point out half a dozen ways it is better than FTP or a USB stick)
All of the criticism on the Dropbox post is addressable, but the discussion would have been a lot more interesting if the original commenters pointed out solutions themselves. That shifts the discussion from a feeling of "This will never work" to "Cool idea! Here's how you can make it better." The latter, and not the former, is important to the culture of HN.
You're right, it is important to remember to give credit.
But for many of the people here, getting something "right" is an explicit and fundamental part of what we do. It's a specific, on purpose focus. So it's natural (and good) that we'd think that way.
