When I offer a critique there are two things I keep in mind - 

I try to work out what the photographers vision was when they created the image and interpret the image based on those thoughts. I write the interpretation as an aid to the photographer so they can see if my interpretation is even close to what they were thinking. This helps them to know if they pulled off an image that will convey their vision.

Secondly and least importantly I look at the technical aspects of the photo - I only comment here if I think that changes would help interpret the vision. Just because something isn't completely sharp or doesn't follow the classical rules - who cares? I don't think that is what is important. 

How you critique is obviously up to you, but you might consider this approach to help you develop your own vision.

If you would like critique of your work by me and many other people, a good way (certainly not the only) is to join Google+ (think of it as facebook for adults) and join Colby Browns Behind the Lens Photo Critique circle. Or head off down to your local photography club (it isn't called a camera club is it?).