Jorge's famous last words not withstanding,
I critique for a living. In Real Life I coach the nation's biggest talk show hosts. And while it's not photo or art critique, there is something very important that applies here.
When you are photographing, you are expressing yourself from a very internal and deeply personal place. Or at least you'd better be. It's the place where "the voice inside your head" lives. Where "the little kid in you" still lives, the place where you hope from.
So to critique work or expression that comes from that place is a very delicate thing. You have to make it all about making a person's strengths and brilliance shine even more, get even more light, while managing the things they're not so good at. I kind of take the approach that everyone has talent, but that everyone also gets in their own way much of the time, and my job is to highlight and focus on the talent while teaching them how to manage the rest.
Thus, a critique isn't really about what's good and bad. It's about what's strong and could be stronger and what's not strong and should be pushed to the back a little more. Saying things like "when you expressed x over there, it was HUGE. Blows me away (and it genuinely must). So I know what it feels like to be blown away by your expression. So over here at y I don't feel the same. Howcome?"
At the same time, you have to comment but NOT suggest unless invited to do so specifically with each comment. Otherwise, you will cause someone to start remaking their work in your image. The critiqued has to say "you're right, I see how y doesn't stand out as much as I thought. What would make it stand out more?" and THEN you offer suggestions.
In my experience, most other methods of critique are either too harsh, too gratuitous, or too likely to make someone strive to please you rather than strive to be better with or without you.