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:
What's so special about Linux?
It's only the most successful open software ever. Otherwise, not much :-P
I recently came across this, which seems to have some evidence in my favor (and some irrelevant stuff): http://www.bakadesuyo.com/2013/10/extraordinary-leader/
Another month has passed and here is a new rationality quotes thread. The usual rules are: