I've received a lot of comments that were 2-3 sentences but still incredibly generic. "I liked this fic. You write beautifully, Bob and John are so cute together! Thank you!" That does not seem like a comment that someone angsted about for half an hour because you could put that comment on literally every not horrible Bob/John fic.

I'm not one of those people who expect super long comments, but I tend not to be happy when a gift recipient can't even pick out a couple of things specific to my fic. Like quote a line they loved and say why they liked it or mention a specific scene they thought was particularly cute or funny or sexy. Especially in a longer fic.