My comments on critiques or posting images of work are this:-
In the unlikely event that images of a 3rd party are posted no form of critique is acceptable, that person did not post their own work so you shouldn't post it either.
If you post images of your own work you should expect a critique whether you want it or not, if you do not want your work talked about don't post images of it.
If you feel the need to comment on another person's work do so politely, an abrupt one or two word answer is not acceptable in actual fact it is downright rude.
If you are serious about critiqueing work ,for everything that is pointed out to be wrong a positive or constructive comment should be left. So for instance instead of just telling someone they have turned a dog bowl (which we have all done in the past)tell the person that you prefer to see a "nice continuous curve which in fact is easier to produce rather than flat bottomed straight sided work." And I have to disagree with burywoodturners, yes it is not nice to have a piece looked at with no comments left but the old saying of"if you cannot say something nice say nothing at all" should stand. Some pieces beg a comment and others do not warrant it. If a piece does not attract comments it is then up to the person that posted it to ask for them (and then brace themselves!!)
Above all else, politeness costs nothing, if you cannot be polite do not bother commenting.