If you don't like a pic then that is personal taste and I would say 'just move on'.
If you think a picture has a fault that curing would improve then that's your opinion. Say so if you wish. But you should explain what you think is wrong and suggest a way of fixing it.
Since the someone who presents a photo that you think has faults (or you don't like) has posted it to a public internet forum, it may be assumed that the same someone wants people to say something about his/her photo. Else he would have kept the photo to himself!
So I think any kind of reply should be accepted, positive or not, unless, of course, the reply is insulting/offending. You should certainly try to avoid hurting the other person, but wouldn't you do the same in real life if you met with a large group of people?
Forums such as this are all about communication. And I think that if you have something meaningful to say then it is good. "Cool pic" as well as "this sucks" are both replies that to me are NOT very meaningful. So even positive replies can be senseless. But a critique that points out what you think is wrong and could in one or the other way be done better the next time the poster takes out his camera can well mean a lot to the poster.
If people don't hear the truth then how can they learn from their mistakes. I have learn so much from this forum that has brought me up to the point I'm at today. That included hearing the whats "not working" in my photo.
In my opinion it's best to be completely honnest, but i think it's more down to how you word things than what you say. I personally wouldn't take any critique too 'personally', because everyone is entitled to their own opinion. Sometimes people lack tact however, and this can be offputting to some, but it's best to just see through it if you're on the recieving end.