Aside from technical considerations, how valid is it to suggest changes to someone else's images? Isn't that just saying, "Your vision is flawed. My vision is superior?"
Criticism does usually not suggest any changes nor does it really judge about a piece of work. We have to differentiate between criticism and recommendations, although both might appear at the same place. This depends on whether recommendations are perhaps expected or custom. One can usually judge the value of an advice by the quality of the criticism.

Isn't all we can say, "My vision matches your vision of a particular image and therefore I like it," or "My vision does not match your vision of this image and therefore I think it can be improved to more closely match what I think it should look like,"?
Yes. However, serious criticism isn't limited to the vision, because serious art isn't limited to that, too. There is usually a context in which some kind "further concept" exists. The critic ideally has to deal with that concept, too.

Even some technical issues are personal. For example, for a particular image, some people prefer a darker printing treatment, some like grain, soft contrast, filed neg carriers, blur, or even out-of-focus subjects.
A serious criticism usually contains:
  • - A description of what the critic sees
    - An interpretation of what he has seen. The more a critic knows about the context of a piece of work, the better he can interpret it. This ideally includes: knowing the artist, his concepts and his other work.
    - A well-founded conclusion why the work is good or bad (or art at all - :-))
    - Finally yet importantly: no one will refuse a critic a personal opinion. However, it should be clear that it is a personal opinion.