The first one is rude, the second one is blunt, the third one is subtle/tactful/whatever.

We keep hitting the Typical Mind Fallacy over and over again :-)

Let me offer you my interpretation: the first one is blunt and might or might not be rude, depending on what the social norms and context are (and on whether thinking about frobnicating the beezlebib does provide incontrovertible evidence of severe brain trauma). The second one is not blunt at all, it's entirely neutral. The third one is a slighly more polite version of neutral. Your fourth example is still neutral, by the way -- there's nothing particularly blunt about explaining why something should not be done (or about using four-letter words, for that matter).

To contrast I'll offer my examples:

• (rude) You are a moron and can't code your way out of a wet paper bag! Stuff your code where the sun don't shine and never show it to me again!
• (blunt) This is not working and will never work. You need to scrap this entirely and start from scratch.
• (subtle) While this is a valuable contribution, we would really appreciate it if you went and twiddled the bogon emitter for us while we try to deal with the beezlebib frobnication on our own.

What's so special about Linux?

It's only the most successful open software ever. Otherwise, not much :-P