These Are Thy Glorious Works

[close] Please ask your fellow author before you critique their work if you feel you will be a good match.
Critique the 'work' not the author and ask what type of critiques they wish for - whether that is a thorough grammar search or just an 'overall' thought on a WIP.
Don't expect people to crit your 20,000 word chapter, unless you have previously come to an agreement.
Fairness and politeness, not to mention professionalism, is the key.
You may 'gush' if you absolutely love someone's work, but please explain why you think it is wonderful. Same when you really dislike something. Explain what makes you uncomfortable.
As a final note, please be aware of 'where' your fellow author comes from before correcting all their colloquialisms and spellings.