When I hit the "critical review" button, I'm hoping for something substantive. (If you've seen my comments, you'll kinda see where my mindset is for "critical"...) If something can be improved, I want to know WHAT can be improved, HOW the reviewer thinks it could be improved, what parts of the piece worked, which didn't... of course I want to know if the reviewer liked the piece, or pieces of it, but if they see room for improvement, I want them to tell me.
That really goes for all my work, but ESPECIALLY for things I mark "critical review." Encouragement is great, but as a writer I'm always looking for ways in which I can improve as a writer and in which I can improve specific pieces of writing. Criticism is great, but I also want it to be specific -- a vague comment saying "this could be better" isn't criticism, it's a waste of your time and mine, because what is a writer supposed to do with that?
There are certainly things people don't need to critique on -- stylistic choices that make a writer unique -- but personally I'd rather have someone critique too many parts of my work than for them to give too little information. As a writer you can accept or reject any criticism you don't find appropriate, but I'd rather be able to discard criticisms than have too few to go on.