In my meanwhile well over three years here on TPF I have learned a number of valuable things, both those that are very helpful to me and my own photography to day, and those that make me consider and re-consider my own standpoint when it comes to view and "critique" other members' photos.
And I have learned that - well according to what Hertz is saying and Aquarium is underlining - as vast as the field of photography is so vast is the range of personal tastes. Some people find photography without people in it boring. Which is why landscape photography will never really fascinate them. Let's stay with those who like people in photos - again there are those that love staged, posed or studio photography, while others much prefer street scenes and candids. The latter will not look at studio pics for as long as they will look at other members' street photography. For example.
Then some like wide angles. And develop their abilities in that area until they are able to take marvellous wide-angle photos.
Others like to filter out tiny things, give them a frame of their own by photographing them, chiselling them out by the use of aperture etc. The people-lovers will find those photos more uninteresting than other lovers of close-up or macro photography.
Again others are all fascinated by colours, the way colours can complement each other or go with each other etc, while again another group loves the abstractness of black&white photography. So it cannot be expected of a lover of black&white to really go for an extensive critique of a photo that only is about colours.
So actually, in the end, the only things that can be critiqued are technical aspects such as under- or overexposure, wrong focus (though that can in the end sometimes AGAIN be a matter of tastes, unless the focus is totally off, and yet ... you never know...

), angles (again ... a slanted horizon can be what the photographer wanted all right!) ... which brings me back to thinking of a thread I once started myself on something like "How objective can critisism be?" or similar, and to the conclusion that it never is objective at all. It is all about tastes.
As to how critique can be formed - well, some may have noticed already: I tend to ask. Put things I am not so sure about in someone's photo into a question. A forum like this (to me it feels like it at least) is a place to TALK, to communicate, and how better can you do so but ask questions? Thus I can call the other photographer's attention to a thing that struck me as questionable and we can talk about it. No need for me to say "This is all c..." (you know) <- just to put out a crass example which you would never hear me say, anyway.