As a reader, I like kudos - don't have an AO3 account, don't want one, but I do occasionally get linked to fic on AO3. And it's nice to be able to tell someone "I liked this thing you wrote, thank you for posting it." without having to make a production of it. Especially since I read a lot of shorter porny pieces where there isn't all that much to *say* - "This was hot!" starts to feel redundant after the first ten people say it.