salt-of-the-ao3:

• if the fic already has a thousand comments, comment still. Your comment will still matter and delight the author.
• if a fic is a decade old and the author hasn’t been active in the last five years, comment still. There will come a time when the author will read and cherish your comment, or maybe it will motivate them enough to start writing again. You never know!
• if the author never responds to comments, comment still. Interaction with the author is a very nice bonus, but you can be sure that even if the author doesn’t answer, they will read it and enjoy it at some point
• there’s no such thing as a too long comment.
• there’s no such thing as a too incoherent comment.
• the author will give no flying fuck about any gramatical errors, typo or other misspellings. If you’re a non-native speaker struggling to express themselves, you can be sure the author will be all the more pleased that you surmounted the language barrier to let them know you appreciated their work. Don’t be afraid!
• there’s no such thing as commenting too often.
• you will never, ever come across as creepy by obsessing over a fic or an author to the point where you worry the author might think you’re a stalker. On the contrary, the author will be delighted by your investment in their work.
• say thank you. It’s always appreciated to see readers acknowledge the work and commitment that is put into writing.
• the floaty review box (ao3 add-on) is your friend
• be positive and encouraging. Positive reviews make writers all warm and glowy from the inside, bashing plunge their soul into icy darkness. You want the first, not the second!
• whoever you are, if you read their fic, YOU are IMPORTANT to the author. Let them know you’re there!

(if any author wants to contradict one of those rules, please let me know!)

(Submitted by @randomishnickname)