I leave kudos on pretty much everything that I read and enjoy. Then I will also leave a comment if I have something more specific to say -- if I want to call out some turn of phrase or if they made me think about characterization differently or...basically, if my response to the story was more complicated than just "yay!"

And I love getting kudos, as a writer; I tend to assume when the email its my inbox that the reader has just had that experience with my fic. They read it, they feel good about reading it, they want me to know they feel good about reading it. Yay!