@ Lic: It's just insecurity manifesting itself, especially in younger writers. Writing is very personal to most people, and when they put it out there, they expect something more than silence. There's little else more discouraging than getting no attention for your work whatsoever, especially if you only recently started writing.
My main point is simply that I think you flat-out lose more if you post it all at once. There's pretty much no advantage to throwing it all out there. Yeah, some readers only read completed works. But your fic will be posted in its entirety eventually, because you already finished it, right? In that case, you'll get those readers when you finish posting the last chapter. No loss there.
The only advantage to posting it in one chunk is to get it off your chest. If you think that's worth the potential loss of attention(?), then sure, go ahead.
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@ Octo: It still takes a lot less time to check well-reviewed fics for quality than it does to check the non-reviewed fics for quality, if only because there are fewer well-reviewed fics than non-reviewed fics.
I will also say that well-reviewed fics obviously have something going for them. Yeah, a lot of them flat-out suck, a lot of them are mediocre, but the author must have done something right to get that much feedback period, whether it be humor, porn, characterization, whatever. I would personally never have read "The Fifth Act" (and held off on it for months) except that it was recommended everywhere and well-reviewed, because time travel isn't my thing, and the first chapter was mediocre (IMO). It was great, though; I'm glad to have read it.
As for FFN: It's just much more well-known in the fanfic world. If you search "fanfiction," it's the first hit, while AO3 is buried in the middle of the page with a name that doesn't make it sound fanficcy. So FFN's where people go when they first start writing fanfic. It's like dA, except for fanfiction. Also, if I'm not mistaken, AO3 still requires invites?