I have phases of being interested and not interested in My Little Pony; I’m mainly here because this site is designed so well. I’m also on YouTube, Offprint, and Pillowfort.

Downplay your criticisms. This one is hard, and it may seem unfair to you, but remember that fan fiction has a drastically different purpose than professional writing. It is for practice and experience in hammering out thousands of words and exploring our own abilities in a sandbox environment. Remember that you are not the author's literary agent or beta-reader, unless they ask you for your services, so don't act like one. What a fan fiction writer needs most of all is encouragement; if you spend most of your time talking about what they did right, rather than what needs fixing, they will be conditioned positively to explore ways to improve upon their strengths, possibly to the point where their shortcomings become forgivable, or they figure them out for themselves and work to fix them. Harsh criticisms generally cause authors to stop writing altogether because they fear that anything they try to do will end up wrong. This is not the effect you want to spread, because it is only through experience that a writer becomes better. Harsh criticisms only tell the writer what not to do; experience tells them what to do.

Do not intentionally sabotage a writer's spirit with malice you have cleverly disguised as constructive criticism. Do not comment at all unless you can honestly tell yourself that you are on the writer's side and actually want to see and help them improve.