An author does not expect a summary or a rational analysis of his book and would even prefer you not to attempt such a thing. He expects only that, while maintaining the greatest possible degree of ambiguity, you will tell him that you like what he wrote.
-How To Talk About Books You Haven't Read1, Pierre Bayard (emphasis mine)
The feedback that one gets when publishing fanfiction on the web can usually be sorted into two large categories: the short comment, and the (much longer) review, usually either 'constructive criticism', or squee with some coherent comments to buffer the caps. Of the two it's obvious which I prefer-- which most people would prefer-- not least because the latter tends to be more specific praise, as in "I liked this [quote] very much, because", rather than the generic comment that can likely be applied to anything, written or otherwise ("nice!" "I like this!" "great job!").
While- personally- the short validating comment is an ego-booster, it generally isn't something I'd come back to. It's nice to have your writing appreciated, but it's much more satisfying to know that your writing's being appreciated for a
specific reason.
While I generally try to give longer and more detailed comments on writing that I like, I am also often guilty of leaving feedback the quick and easy way ("Great writing, loved this, thank you"), or worse, leaving a glib review. (Personally, I
like rambly reviews, but maybe it's just me.) On delicious, with the added option of surreptitiously bookmarking something, I usually add comments that I wouldn't necessarily tell the author, notes that remind me that this fic is typo-ridden, but is otherwise good, this fic has characterization issues even if I don't know the specifics.
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1A note about the book from which the quote's from: it is not a parody of the zombified classics sort, but an argument for "non-reading" and, like it says on the tin, being able to converse about books one hasn't read. There are many ways to have 'read' a book, Bayard argues, using terms like 'the inner book' (the meaning derived from the book by a reader), 'the screen book' (the impression held of a book by a reader), and 'the collective library' (books anyone with any sort of literary cred is expected to have read). The book attempts to make a case for non-reading and criticism independent of material-- since every reading is bound to deviate from the author's original intention, he argues, meaning derived from reading may not be crucially different from meaning derived from non-reading. [a caveat: I read this book a good half-year ago and saved a couple of quotes, so most of my interpretations are probably more than a little off.]