I always try to leave comments on things like fic. I have a friend who loves the kudos button because it means she doesn't have to say anything. Her reasoning is that she usually doesn't feel she has anything meaningful to say other than "I really like this!" which is basically what the kudos button is anyway, I guess. If that's all I have to say I generally try to pick out a quote or something that really stood out to me so the author knows why I liked it.

But I totally agree on the liking questions.