I only ever posted a WIP once and it was a very short one in a small fandom, so sometimes I'm not sure if authors of popular fics in bigger fandoms even remember the names of their commenters, but when the audience is smaller it's more obvious.
(I was confused for a moment because I remembered you have multiple chaptered fic, until I realized that multiple chapters can be posted at once. It's Sunday morning.)

Leaving longer comments for me was a matter of practice - and sometimes I just resort to copying my favorite passages and keyboard smash, but I think it gets the point across okay ^^