I just left a classroom talking about Marshall McLuhan's "the medium is the message", which in conjunction with this thread
has me thinking about the 'medium' and the 'message' of fanfic (or, I guess, 'fanfic as it operates on AO3').
Basically, I think fanfic has made great use of the freedom of digital publishing, better than ebooks have because the publishing industry tries to keep ebooks roughly equivalent with published books. You can post a 1,000 word sex scene, or an 800,000 word epic, and there's no need to make it the "right" length to fit on a bookshelf, or the "right" length for the price of an ebook.
This also makes me realise that fic doesn't advertise itself with covers and graphic design "branding" the way that print books and ebooks do.. in literature classes I'll often show students many covers of 'classics' like Tess of the D'Urbervilles
and invite them to analyse how each cover 'sells' or interprets the book (and what they think is most accurate), but fic advertises its contents/genre by the summary and the tags... which also seems to let each fic kinda stand on its own, you don't have to figure out which 'visual language' will sum up your fic, you can just say "60k, hurt/comfort, frottage" and people try to figure that out.
I haven't really finished this thought but please opine on the 'structures' of fanfic reading! Does pro fic feel so different because it's actually written differently, or just because it's consumed in such different ways??
 McLuhan tl;dr: “media are not just passive channels of information. They supply the stuff of thought, but they also shape the process of thought” -- aka what's important about printed books is that they force you to sit alone in a room for a very long time concentrating, no matter what the book is about, and what's important about electronic/digital media is that they don't
make you do that.