Criticism is just an honest opinion and people who are easily offended by it forget that when they post a pic on the Internet for all to see. It doesn't matter who you are or how good you are at painting, people will always have an opinion.
On the flip side, giving your opinion is difficult when you can't see the person face to face. That way, you would be able to judge what level your criticism can be pitched at to help someone improve and not invite unnecessary negativity.
I studied architecture at university and have had many 'crits' where months of work is torn apart in front of you. After much wailing and gnashing of teeth it does make you better, if only as a self preservation mechanism to not be in that position again!