On the whole I think that most people commenting on a picture do show respect and awareness of the subject's feelings and emotions. However sometimes a comment is made that reveals a lack of regard for the person in the image.
Classic examples are comments which turn an attractive woman or man into a sexual object, or mock, trivialise or patronise the subject.
Even if the person in the photograph will almost certainly never see the comments made on their image, I believe it is of fundamental importance that comments are respectful and that you only say what you would be prepared to say looking that person straight in the eye.
The internet is vast and people on your screen can appear somewhat abstract and removed from the real human being, but each person you see in a photograph has a life, aspirations, hopes, fears, relationships, experiences and feelings just like you or I.
Of course the respect for a person's feelings and dignity does not only apply to commenting on pictures of people, it is also fundamental to the ethics of photography. When we lift our cameras to take a picture of another person we have to be aware of their humanity, the ethics of what we are doing and our subject's right to dignity and respect.
I'm sure the people reading this share my feelings and as usual I'm preaching to the converted, but hope that you will appreciate me putting the focus on this subject, which is not often touched upon in blogs and articles.