When I critique, I usually try to 'suggest' instead of saying what is wrong or whatever... There is no such thing as 'wrong' when it comes to art. Its a matter of interpretation and if I see something that I personally think would make the image look better, I'll try to 'suggest' (strong emphasis on that word) what the artist could do to create that. Quite honestly, if I don't have something in the image that I feel I can critique based on my (may it be) limited knowledge, then I don't leave a comment. By joining the critique group, I would assume that each member is looking for ways to improve and not more 'oooh's and aahh's' that in my opinion are useless when you're still learning.

If someone posts an incredible image that really strikes me as good, I will tell them so...

I actually had a situation where I felt a comment posted was rude/sarcastic... but after speaking with the person I realized I had over-reacted and I believe we smoothed everything out... If you really feel that someone or several are flat out being rude, why don't you try talking with them and see what it is they really meant?