I used to do that, both in real life and face to face. I was a guest lecturer in what is known here as TAFE (sort of tech college) and also was often asked on various cartoonists sites to offer opinions on newbies work.
At TAFE it came with the territory, but on the Internet it was getting to the stage where, instead of people being repelled by someone as opinionated as me, I was sought after. I even got emails (still do) from people asking for critiques on their work.
Two things stopped me from carrying on with this pantomime: One was that I have always been aware of my own failings and inadequacies, and never really felt comfortable pointing out other peoples'. The other was that it proved to be the most futile and irritating exercise. Nobody but NObody ever accepted the advice graciously, honestly and with any desire to actually improve their work. Almost without exception, the recipient would say "yes, I already did that/know that, this is just a rough draft/preliminary sketch/beta test, I'm going to finish it off later prior to submission".
That sort of thing. Even with a polite "thanks for your input, Malc" you could tell it was typed through gritted teeth, the keys pounded within a hairsbreadth of their tolerance.
People don't really want to hear anything other than "this is the most brilliant piece/drawing/writing/design/concept I've seen in many a long year. You needn't do a thing to it, any publisher/network will snatch your hand off - make sure you engage a first class lawyer to guard against copyright theft!"
I could write a book on this stuff.