Everyone was encouraged to also write constructive criticism for fics that had serious flaws, of course, but the main pressure that I saw regarded the great fics, the fics you love, the fics you've read half a dozen times or more, the fics that deserve some applause, but applause can't be heard over the Internet, so you write a bit of feedback to let the author know how much you appreciate their work, and that in turn encourages them to write more.
I felt this Feedback Pressure as a fic author, of course, wanting support and encouragement from readers, but I really think it was ingrained more through being an appreciative reader (after all, I've read a hell of a lot more fics than I've written), especially on LiveJournal and FF.net, where some authors tend to include a bit of commentary regarding their writing process and their hopes for reader response, and where readers/responders sometimes engage in meta-conversation/debate on the topic in the comments.
As a result of this feedback-encouraging culture, I got into the habit of nearly always stopping to write at least a sentence or two to the author if I'd enjoyed a story. Often, they responded with joy and gratitude at the few words of praise I'd taken the time to send, and it's always great to be able to make someone else feel that good, so it caused happiness all around.
But it's only today that I realized that it's spread outside fanfic for me.
Yesterday I was putting on the dichroic glass pendent I bought back on Etsy in January, and which has become my favorite piece of jewelry, and which has also inspired several different strangers to compliment me on its beauty over these past several months. And as I was fastening the necklace around my neck, I thought, "I should email the woman who made this pendant to let her know how much I love it and how much everyone around me seems to love it, too. She'd probably like to know that."
So I wrote her an email yesterday, letting her know how much I like the pendant after 5 months of owning it, letting her know that it has become my favorite piece of jewelry, blah blah blah. And I got email back from her this morning & she was just blown away, and I realized it's probably not all that common for a buyer to contact a seller several months after a purchase to let them know how much they and their work are appreciated.
A few hours later, I was walking down the street, and I thought, "Well, yeah, but I tend to do that. If I see someone wearing a beautiful skirt, I'll tell them I think so. If someone's hair is an amazing color, I'll tell them I think so. If someone posts a great video to YouTube, I'll tell them I think so." And then I paused, and I thought, and I decided that it all started with fanfic.
I think I was shyer about giving people compliments before I ventured into the world of fanfic, but fanfic taught me that there are very few people in the world who will object to being told, "I think you (or your hair/your skirt/your music/your video/your writing/etc.) rock!" and, in fact, most people really like it. Sometimes it makes their day. So if you're thinking it, why not say it, right?
So yeah. Fanfic made me a nicer person. Or, at least, more willing to voice my nice thoughts to strangers.
Caveat: If I see someone under the age of, say 22, and I think their clothes or hair or whatever are really cool, I often decide not to say so, because a couple times when I've done so, the young person looked at me like "Why is some old lady liking my look? This look gets the Old Lady Seal of Approval? This is not a good thing." And I decided that a youngster might not take it as a compliment if I think they're fabulous, because they only see an old lady when they look at me and don't see how fabulously cool I am on the inside & what amazing taste I have. So I often choose to spare them my confusing compliments, unless the coolness is so extreme that I figure anyone that cool wouldn't possibly object to compliments from old ladies. Because truly cool people think other people are cool, too. :)