Constructive criticism isn't telling someone to go educate themselves or calling them names in a WIP thread when the OP is trying to create something new or is trying a new idea or method. It isn't constructive, it doesn't help with the creative process in any way... what it does do is run people down for no good reason and for no purpose. Telling someone their item/figure/image is garbage/looks like c**** isn't helpfull... Telling someone to do things a specific way and only that way and insulting them or putting them down when they won't isn't helpfull... Making unreasonable demands and ranting when no-one will give in to them is not helpfull... There is nothing "constructive" about it

Constructive Criticism shows both good and bad points and provides suggestions on various things that could be done... it allows for room for more than one preference or opinion and also allows the person that asked for the critique to have his preferences as well. Constructive Criticism also is respectful of the idea/effort the person is making and is not simply a list of demands or instructions saying they must or they have to... but most importantly it is helpful and constructive and it acknowledges the good parts of the work as well as the bad...

There is a vast difference between constructive criticism and bashing... and much of what happens these days in many forums is bashing plain and simple... to try and claim it's anything else is ridiculous!