I enjoy commenting and critiquing and I'll be honest, I tend to be a bit harsher on those who are clearly good photographers and/or experienced! I think it's about moderating your critique to be of benefit to the person taking the photo. Saying, "lovely shot" when it's clearly not is probably less helpful than not even commenting! (Bearing in mind we all see different things in photos and what one person likes another might not). If I think a shot could be improved by doing something differently, I'll usually suggest it - in a gentle way if I don't know that person from last year's 52. Of course, if it's
@Dark Knight or
@posiview etc. then it's open season

Being overly critical of someone who's new to photography is just as likely to put some off, so personally I think most of us try to avoid that. Time is tight though, so if it is genuinely a good shot then clicking Like or posting, "great image" is totally fine. Especially if you can't think how to improve it...
There are one or two here though who are new to photography but whose images are really very, very good - I'm suitably impressed!