I think this works (unfortunately) just as a agree-disagree counter... To vote "fair" on a critique that is pure and honest but one disagrees with it seems strange an unnatural for way too many people.
To critique involves passing judgement. To be judicial one needs to be impartial and aim at considering all aspects of the work, good and bad. That is different from giving equal time to what is good and what is bad, for there may be more bad than good or vice versa. Usually there is more bad than good, like the way entropy works.
If we are remaining true to the idea of criticism, we should avoid knee-jerk opinion. In some sense all of our reasoning about art may be 'subjective' yet that doesn't mean we should cease to have our own objective take on art. The alternative where art can be given no objective assessment is a frightful alternative, one which some people assent to. But if art has no objective assessment on some view, we are basically being nihilistic about its potential. If there are better ways to do things -- which one can profitably assume -- then we are granted the conscious ability to refine and improve our craft and visionary expression.
If someone asks for a critique then they should accept whatever the person suggests (unless it just something downright nasty). To me it means that another person has been generous in using their time to provide feedback from their perspective. Giving feedback not only provides the artist with information it also allows the person who is providing the feedback to learn too as time has to be spent considering a whole range of factors.
Long version: To me, you can either miss your mark, hit your mark or exceed your mark. If they miss it, I point it out and explain/give advice/etc. to the best of my ability. If they exceed I compliment or admire it. If they hit it then they simple did what was "expected" of them - it's not bad or great just average.
I say that because I think there is this weird thought that in order for a critique to be fair/good there must be compliments in it. I have seen people go as far as saying "For every one bad thing you point out you must say two good things". I just think it's wrong to expect compliments, which is what a lot of people do.
Non-biased and constructive. Pointing out plausible flaws and giving advice based on factual experience on how to improve and not resorting to "I don't like it because this and that" is what I call a fair critique. Not liking something and thinking something is bad are two different things.
I guess constructive criticism usually has a positive tone as opposed to condescending (given that negativity doesn't help much) but I think "If I mostly agree with the author's opinion" is the only one that I don't consider at all. All opinions provide some insight, so presentation is really all that separates it. I voted "nice" as being the factor but I just want to clarify because it's maybe not an exact match for my answer.
I think good critique ought to be...well, a critique. The author of the critique ought to point out what works, what doesn't work, mistakes, etc, but also give advice and suggestions on how to make the work better. I see critique as something that's supposed to help the artist improve, and so critique that's made up of praise doesn't seem like something that would help very much. There's nothing wrong with liking something about a piece of artwork and admitting it, but nothing but praise doesn't help the artist at all.
Going through my previous critiques, I noticed no matter what I write there are always several people who consider my critique unfair. Overall it seems more like an "agree - disagree" counter to me, but maybe I just have a different notion of 'fair'. So what do you think?