Yes, it can be very tricky!!! I'm quite glad that there's the option to leave kudos on AO3. It's something that is missing on LJ and it makes it easier to let the author know that you appreciated their story, even if you cannot put that into words, for whatever reason.

Oh yes, I definitely think too that a comment is more appreciated. But I remember those endless discussions on LJ about the amount of people who read and never commented - I think things have improved a bit now. But comments are definitely the best - after all, the author has put a lot of time and effort in the fic and made it available to us all to read and enjoy. :)