That where such analogies to fandom fail. No one posting fanfic is forcing anyone else to read it. Fen tend to be pretty careful about labelling stories so no one has to read stuff they don't like. Now if someone did put a metaphorical gun to my head and force me to read crappy fanfic, and was then dumb enough to expect me to respond with thanks and praise...I'd be likely to give my honest and scathing opinion. 'Cause holding a gun to someone's head (even if it is only a metaphorical gun) = not polite.
But inappropriate analogies aside, I want to talk about constructive criticism.
Let's start with now I feel about concrit on my own fic. I can sum that up in three words: bring it on! It baffles me that this is even a question. Of course I want concrit! I welcome it, I devour it with a spoon!!! ...but I don't demand it because I recognise the effort involved in offering it.
As a writer, it's not easy to take concrit. My initial reaction when someone is critical of my work is generally defensive. It does take time before I can step back and look at it objectively. Dude, you've just told me my baby is ugly. Even if it's true; I don't have to like it.
Back when I was in SG-1 fandom, someone was brave enough to take me to task for the way I'd written the aftermath of a rape in one of my stories. She did it publicly, on a mailing list of which I was a member, a list created for the purpose of such discussion. Everything this person pointed out was totally right. I had reasons for writing those scenes the way I did, but I hadn't realised just how badly it came across. For example, I had Janet commenting that what happened to the victim wasn't really rape. I was well aware that her statement was inaccurate and unfeeling: it was supposed to be. But this person pointed out that Janet's a medical professional and was speaking in that capacity, so for her to take such a position was...well, I forget the exact words used. But not right. Fair enough.
She had other criticisms of the story that I didn't agree with. One thing I remember was her criticism of a sex scene that included light bondage; she felt the way the scene was presented didn't reflect real BDSM. Well, no, of course it didn't. Neither of the characters was experienced in that, neither of them understood the rules of BDSM the way someone versed in the scene would understand. Of course they "did it wrong".
I didn't go back and change the fic. No, I'll let it stand in all its bad-fic-idness. But I also never made that mistake again. And if, someday, I come across a web page or journal entry ripping my story to shreds for that catalogue of mistakes...it'll be no more than it deserves.
But one of the reasons concrit has such a bad rep is that so many people don't know what concrit really is. To give good concrit, you need to understand what a story is trying to achieve. The most important part of "constructive criticism" is the "constructive" bit. To be constructive, you must understand the author's aim.
What if I'd written that story with the intention of showing how badly a rape victim can be treated by the authorities? What if the whole point were to make the reader boiling mad that a doctor could say such a thing? It wasn't - I screwed up and I admit it - but what if it were? Then it would still have been valid to criticise my characterisation of Janet (in most SG-1 fanfic, she's a candidate for sainthood), but smacking the story down for achieving what it set out to achieve is not inherently constructive.
That's a very fine line to walk and very few fen know how to walk it. I include myself in that. I don't get it right all the time. I remember beta reading a story a couple of years ago which began with a man (a canon character) destroying evidence of a murder. My own preconceptions of that character were so strong it didn't even occur to me that he might not have been the killer; and my comments to the writer were based on that (incorrect) assumption. Bad beta, no biscuit! But after she explained it to me I could read the chapter fresh and see that it was my mistake, not hers. She had included several things that should have signalled to me that he hadn't done the deed. Perhaps she could have made it more obvious but the error was mine.
I've received a number of comments over the years which essentially tell me my fics would be vastly improved if I conformed to the reader's fannish preferences. This is not concrit. It's not constructive, because I write my worlds and characters as I interpret them from canon, not as someone else does. It's not even "criticism", really, because it's essentially saying "Here's how I want it, if you don't do it my way you are wrong."
I'm talking about comments like "I really never read het, but I read this anyway. I enjoyed it, but it would be so much better without all the icky het sex." (that's a paraphrase).
I have no idea how to respond to that kind of comment. Generally, I don't respond at all. What am I going to say? "Gee, I'm sorry, but I didn't hold a gun to your head and force you to read my icky fic"? 'Cause, really, I didn't. Not even a metaphorical one. And the weirdest thing is these kind of comments are nearly always phrased as if I'm supposed to take it as a compliment.
But I'm not solely a writer. I sit on the other side, too. I read and I beta and I know that giving concrit is really, really hard, even when you've been asked for it. How do I explain to someone that I think this story could have been great, and I see how hard you worked on it, but I just don't recognise these characters as [whoever, from canon] and here's why...but maybe you could fix it if... How do I say that and not come off as a real bitch?
Worse is when I read a story I've been asked to beta and my only reaction is something like: WTF!!!??? It's happened to me a time or two. I know what I should do: politely explain that I'm not the best person to beta this fic. But I'm really bad at doing that. I'm scared my friend will ask why and I'll have to tell the truth.
Unsolicited concrit is a whole 'nother kettle of fish. There are times when it's just totally not appropriate. If someone specifies they don't want crit, that's fair enough and I think that should be respected (note I'm talking about communicating directly with the author; not about reviews and recs). When someone says something like "this is my first ever..." or "please be kind" - that means, to me, that they don't want concrit. I might choose to pass on that story, and very likely if I do read it I won't comment at all, but I won't concrit against the author's stated wish.
Concrit is not easy to give and it's not easy to take. I don't, honestly, think I'm very good at giving it, because I'm much better at picking out the things that need fixing than I am at the "constructive" part. I am also not good at recognising that maybe I'm the one missing the point. Consequently, I rarely offer concrit unless I'm asked for it, and even then I'm wary.
To be able to handle concrit, a person has to be able to admit that their writing isn't perfect, doesn't work for everyone and can stand improvement. That really shouldn't be so hard...but it is, isn't it?
Here's the thing. As hard as it is to take real concrit, it's so much harder to be the one giving it. And, as a writer, I take it in that spirit. I welcome it. To put it another way, I second everything kita0610 has to say right here.
On another, shorter, note:
Fic reviews are not concrit. Concrit is not a review. Both are valid, and both have a valuable place in fannish discourse. But they are not the same thing. In this latest fandom kerfuffle, a lot of people seem to have them muddled up.
Recs and reviews are not written for the benefit of the fic writer(s). They are for other readers. A person might review a story in order to tell others it stinks; that's her right and she can be as mean as she likes. She can point-and-laugh. She can joyously lampoon the disembodied zombie hand of Rob Lowe and anyone commenting on her review can do the same.
The author being criticised can't do a damn thing about it. If you are the author and you don't like it, don't read reviews of your fic. You're not the intended audience anyhow.
A reviewer has no obligation to be constructive. That's not why she's writing. A reviewer has something to say about the story. It might be bad. It might be mocking. It can also be gushingly wonderful praise. Good reviews do exist. Fans like to pretend they don't, so we give good reviews a different name: we call them "recs".
In my own recs I try to say what I liked about a story and not mention its flaws unless they really bug me. If I didn't like the story in spite of the author's awful spelling or weird characterisation, I wouldn't be reccing it. I'm not writing reviews; I don't have time. A review takes a lot of thought and effort. I'm just saying "hey, guys, here are some fics I enjoyed."
If I were writing reviews, I would point out those flaws. I recced a story a couple of weeks ago that was in an LJ with a blindingly orange layout - it was impossible to read. (format=light is my best friend!). I didn't mention it in my rec; in a review I would have suggested that dark glasses might aid the reader. And possibly the author thinks her layout is the coolest ever and would be highly offended by that. If so...well, that's the way it is. A review is for the benefit of the reader, not the author. Always.