listen i love an ao3 comment as much as the next person, but a thing that still often gets glossed over in the (understandable!!) clamor for folks to leave comments is that if you say something negative or critical in one of them – without being asked – even if it's tossed out carelessly after something nice(r), and that author is actively writing or planning to write something – for free – there's a very real chance you just stopped a new fic from being written, dead in its tracks, creating the same result as not leaving a comment, because it's playing with the same ego
(this isn't like a totally abstract fanfiction concept either – if a dozen people like your new haircut or your outfit, but one person says something negative, it's human nature for many of us to only fixate on the negative comment)
(and miss me with the concrit is important – this is fanfiction, some of us are just sharing the stories in our heads, not working toward our novels, and even then, if you get something for free and you insult it, even casually, you're rolling the dice on whether you'll get more, which is your right, for sure – seems like a high risk, low reward scenario to me, if you liked the fic enough to say the positive thing in the first half, though)
source: the 2000 words i have already written, the comment i received while checking my email, and the google doc i just said well fuck this, and closed lol
