In my opinion when someone allows their feelings to be hurt by a comment its because deep down, some where inside they feel that comment is the truth. Is this an indication of oversensitivity, no. In regards to the internet people are far more likely to chose words left carefully online. They aren't face to face with you. So if someone says something that would already have hurt your feelings as it is an insecurity of yours and words it in a manner that is very blunt it more likely to hurt your feelings more and I wouldn't say you're being over sensitive either.

It runs into over sensitivity when you begin to interpret all comments as negative ones even if they weren't necessarily meant that way.

Example: I say in an email, "Oh my gosh your outfit today made you look like my grandma, we need to go shopping!"

An over sensitive reaction would be to assume I meant that you currently looked like crap and I thought your style was horrible. In reality that would be just sort of a jesting between friends.

Several things that might help.

Has the person always responded this way, or is it a change from previous behavior?

Is it only when responding to YOU that they seem critical, or they also critical to others?

Don't respond right away. Take a break, do something else, then re-read their reply.

Ask someone who is not involved to read the post and give you their opinion.

Take the conversation off line and ask them in a private message if you have done anything to upset them. Tell them that you are concerned that it seems to you they are always critical. Give them a chance to respond. Use neutral terms.

Remember, the internet is the land of trolls and flamers. Everyone has to make an effort to restrain themselves from boorish behavior, and it isn't always easy.

Here's an interesting perspective:
http://www.youtube.com/watch?v=YOqKyrqPgDk

I would assess how this person treats others; are they picking on you alone, or do they act badly to many people?

I've participated in a few online forums where certain people made me feel disliked or even mildly attacked, but over time I realized that most forums have their "queens" who like to rule their little territory. Often they accomplish this by assembling a small group of lackeys who support them as yes-men/women, and will invariably share their attitude to outsiders. This gang behaves like the "mean girls" in school, and it's obvious that their juvenile actions reveal emotional immaturity.

When I was a newbie on these forums, my tender feelings were hurt, because I couldn't see why they treated me like an interloper. I stopped going to two of the forums, but I still use one for professional reasons, although I mainly lurk and rarely comment.

Eventually, I found a forum which is exceptionally warm and welcoming, and it's members behave in a civil manner. This reassured me that it's not my fault, and I wrote off the other experiences as examples of childish behavior.