AO3 Floating Comment Box made it so much easier for me to leave comments on fics.
Lots of times I have just not commented on fics that I really enjoyed, because I couldn't think of something to say in the moment, or didn't have the social battery to even leave an incoherent keysmash.
With the floating comment box, whenever I read something that moves me (joyful, horny, funny, angst, fear, amazement, whatever), I can select the text, add it to the comment box (with quote formatting) and then drop in a comment or even just an emoji.
Then the author at least knows that those particular words caused a reaction in me.
I think that's a thing authors like...? Anyway I'm sure that most authors would like it more than receiving no comment at all, which was my previous default behaviour.