This is something I wrote for the Digital Photography School to help new photographers learn how to critique their own and other's work.
I know we have a lot of people here that might like to take a stab at commenting on shots posted on the forum, but feel they don't know enough or are unsure where to start. I, too, was nervous about doing this, but I've found that by studying the images of other people and analyzing what I see, I've been able to improve my own photography with self-critique. It's really a great learning process!
Here are some of the things I look for when I critique a shot. Of course, not all things apply to each image and there are always exceptions to "the rules".
1. Is it in focus? This is actually a more complex question than you might think. Sometimes an OOF shot is what the photographer is trying to do (such as showing movement), but generally the subject should be in focus even if the surrounding area is not. Notice the depth of field and see if it is working well for the image. Would it look better with a narrower or wider DOF?
2. What is the subject? Is it apparent when you look at the picture? Are your eyes drawn to it, or are you asking yourself why did they take this shot? If it has to be explained, it probably is lacking a purpose.
3. How is the composition? Are there extraneous areas that are not adding to the overall effect? Is the subject lost in a sea of background? Not getting close enough to your subject may be the problem. Maybe cropping will save the shot. Maybe changing from horizontal to vertical would make the difference. Is your eye moving easily around the image from subject to secondary subjects (visual rest spots) in a visual path or are there abrupt stops? Would following the "Rule of Thirds" improve it?
4. Does it have impact? Are there any interesting elements, such as perspective, dramatic angles, leading lines, colors, textures, shapes, etc? What makes this shot stand out from the usual? What grabs your attention? Anything in the shot that helps set the mood?
5. How is the lighting? Are there blown out highlights or areas so dark you can't make out details? Has poor lighting created shadows that distract? Can some post processing help? Has the direction of the light hitting the subject helped or hindered?
6. What is the mood it evokes? If you still feel uncomfortable commenting, particularly on a technical level, mention how the shot makes you feel. Discussing it on an emotional level is just as valid and important. Remember, technical or emotional, it's all opinion anyway.
7. Offer a solution to the problem if you can. Do you know some editing trick that could help? Can you give a suggestion that would fix the problem next time?
I usually start out by saying something positive, and there is always something positive, about the picture, and then pick out something that I think could be improved. If I can, I explain what they could do to fix it or what to do next time so the problem won't happen again.
And a few rules for people asking for a critique:
1. Include pertinent information: camera settings, time of day, perhaps what you were trying to achieve.
2. Asking a specific question, such as "How can I improve the lighting?" and explaining what lighting you used will get you the precise answers you need to improve.
3. Saying "I love this picture! What do you think?" will not get you the help you need to improve. Better to put those in the "Share Your Shots" thread.
4. No comments on your picture? As forums get busier (and remember both mods and members have lives outside of DPS!), your thread could get overlooked. Try giving it a title more interesting than "Critique My Shot". Ask a clear question in your post. Get involved critiquing other member's images and join in discussions (like polls) and games (such as Photo Focus/Topic or True or False). Before you know it, you will become an active member and people will go out of their way to look at your posts. Additionally, take a good hard look at your image. Is it lacking a lot of the things mentioned above? It could be passed by because it (and I know this sounds harsh) is just a snapshot. Posting your best efforts will get more comments as well as help you learn how to critique your own images.