Part of the photo sharing aspect of services like Flickr is that you get to comment and be commented on. A few years ago I was an avid supporter of photo.net but tired of the "same old same old" content and, to be honest, the often brutally offensive remarks that strangers made to each other from behind the screen of an internet connection. I used to wonder what they might say to each other if they should ever meet face to face!
Flickr has plenty of comments but, to be honest, they are too bland much of the time. It is nice to see people react with a positive, of that there is not doubt, but simple "Wow!" and "Ooh!" don't really cut the mustard if you are looking for constructive criticism. But then I guess most Flickr users are not looking for criticism, and too much negative criticism might turn would be photographers away from the medium.
Which is why I believe a comment should try to include a reason why the comment was made, not just a simple congratulatory grunt. And if you can't come up with a reason, then just click the "favorite" button!