I remember back in my university days my lecturers were quite harsh when they critiqued our works. One lecturer could even manage to bring adults to tears. It was a harsh reality but as we were being trained to be 'professionals' they wanted to see professional work from us (impossible when you're a first year student) but it at least showed us what we needed to aim for with our work.
Although I really like Ricktas' idea and critiquing style, not everyone on here is looking to be a professional (I'm assuming) but I'm sure we're all looking to improve (even the oldies ).
If you want to critique photos with the guidelines Ricktas has provided then great! Not sure of the benchmark? My advice is to look through magazines and photo books for examples of the benchmark. As, generally if it's made it into print then it's got the goods.
If you don't feel up to critiquing like that or feel you're not that cold (joking) then you could try a much softer but effective critiquing style; the ol' ONE, TWO! That is, one negative feedback and two positive feedbacks (it's always a good idea to make the first and last comments the positive feedbacks). Example:

1. I really like the composition of the bla bla bla. Its position in the photo really strengthens the bla bla bla...
2. The photo is a little under exposed, a wider aperture would have helped.
3. The colours work really well in conveying the energy of the moment bla bla bla...

Try to cover artistic, technical and aesthetic areas during the critique.

This system doesn't always work as there are sometimes more bad than good but it might encourage more people to add comments and not all of the negative remarks would be coming from one person.

Just another idea.

Cheers
Matt.