Actually there's real learning to be had from providing feedback. It makes you consider the image, what in your experience you might have done differently.
Some people concentrate on the technicalities, is it sharp, was the depth of field correct or would it have benefited from more/less, lighting, shading and then processing.
Others with more artistic flair on the composition, meaning and interpretation of the image, sometimes referencing other photographers.
Sometimes these comments brings forth a discussion the original creator didn't intend or see when creating the image.
What it does do is encourage discussion, which after all is the purpose of a forum, right from roman times
(a meeting or medium where ideas and views on a particular issue can be exchanged).
Quite often I'll ask how it was lit, where it was taken, even what was the thought behind the image, was it planned, conceived or chanced upon etc.
So it's not about all about criticism, it's about the discussion. Through discussion we all learn a little