I think politeness can be a range of possible values rather than a discrete quality. Being polite but also direct is fine, it's just when you start to edge down the spectrum of politeness towards being politic that it might detract from the quality of the dialogue. I do believe that one should still adhere to the rule of thumb: "Don't be a dick" even if one is being direct though.
One of the editing guidelines for Wikipedia is "Assume good faith" (http://en.wikipedia.org/wiki/Wikipedia:Assume_good_faith). It strikes me that is precisely what "normal" people do not do when criticized but what analytical people tend to do, especially when communicating with one another (assume that criticism is not a personal attack or status seeking, but rather is taken at face value). In that vein I think your suggestions are useful and valuable for dealing with regular people in real life or people on the more vanilla internet climes like Facebook, but they might not be appropriate for the frank and analytical types of discussions that take place on sites like LW (and HN).