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.
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?"
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.
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,"?
A serious criticism usually contains:
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.