From the OU course on digital photography:
So, you see guys, saying something is rubbish or worse does not really cut it in terms of critique. Commenting on composition, colour, use of shadows, texture, the taking of an opportunity, highlight detail, good editing, over editing, sharpness, crops....... Those are the things to comment on when giving critique.
Being encouraging is really important, no one likes negative criticism. However it is possible to offer constructive criticism that can help you, and your fellow students, to develop further as photographers.
Look at what you have learned and consider how they affect the way that you view an image. When you look at an image do you look at the composition of the image first, or do you immediately look at the technical quality? For many of us, our immediate response to a photo is intuitive. We like it because it reminds us of something, it has colours that have good associations for us, it shows a place we would like to go to. There is nothing wrong with this response- a good image should evoke an emotional response to engage the viewer. What do you do though when that emotional response is a negative one? It may be a great picture but it has bad associations for you. Even if you are not keen on the subject depicted, you should still try to recognise the good qualities of the picture.
A positive response to a picture is a good start for critiquing it. Consider what you like about the photo and say why it works for you. If your response is negative then you need to think why that is and how to offer some positive comments to help improve the image, or assist the photographer next time they have a similar photographic opportunity.
If there are multiple problems with an image it may be helpful to concentrate on one or two things that can be fixed. It can be a bit dispiriting to have an essay of comments, no matter how helpful they are!
So, you see guys, saying something is rubbish or worse does not really cut it in terms of critique. Commenting on composition, colour, use of shadows, texture, the taking of an opportunity, highlight detail, good editing, over editing, sharpness, crops....... Those are the things to comment on when giving critique.