As someone who runs a rec comm (sawyerficrecs), did a month at crack_van and who regularly recs fics in my LJ, I just wanted to add something. The missing ingredient, if you will.
There've been many times I've heard from someone that they really liked a fic and then I see that they didn't leave a comment for the author. I can't say it enough - authors live for feedback! (OK, there might be a few who've left a fandom or don't like their old fics and don't care about fb but they seem to be the exception.)
I know there are tons of lurkers who don't leave feedback for whatever reason: nothing profound to say, too busy, can't choose just the right icon, who knows? But a simple, "Thanks, I really enjoyed that" is always appreciated.
And don't forget to leave feedback when multiple authors are involved! I coauthored a WIP and found that people weren't leaving fb on all installments. On the one hand, that means we did our jobs and the piece worked well enough that people wanted to just keep reading. Fair enough. But how about when a fic isn't an official series and is being kind of tag-teamed (like the jackjames_verse fics)? I know I'm usually reading all the other fics so I see feedback left for others or myself on a story I didn't write. But other authors might miss it. So spread the love! (And yes, I still owe people some feedback myself. *cough*)
Is leaving feedback really that much work, folks? It doesn't have to be a thesis, just a simple "Thanks" and maybe one or two things you liked about it. I think that's all authors want.
And conversely, it's not just readers who should be leaving feedback. In my mind, there's nothing that takes the steam out of your sails like leaving a glowing rave about someone's fic and then never hearing back from them. Again, an epic poem isn't required, just a "Thanks, glad you liked it."
Feedback is love!