Pretty much what everyone else said. Writing fiction takes a lot of thought and time that could be spent doing other things, so I really appreciate authors who share their work and want to make sure they know it when I like something. Some days that's just pushing the kudos button or saying "I really enjoyed this, thank you" on migraine/pain brain (or just plain brain dead) days.

As for the comments themselves, I've never had a problem stating my opinion on what I think/feel (boy, howdy). Just do that and say why a story (or a particular passage, or mood) evokes that reaction in you. Flailing around and fan-girling/boying isn't always a bad thing, sometimes a story is just that affecting.