Apparently since people on 4chan don't know the difference between constructive criticism and bitching, I made this primer. Do yourself a favor and read it- it could mean the difference between a writer genuinely improving his work in response to your feedback and making an absolute tit of yourself on the threads...again.

All of the below applies to art as well- just replace "fic" and "writer" with "art" and "artist". 

Be Specific

You might think that a writer can automatically detect what part of a fic you think is shit just because you tell them it's shit. They can't. If you're going to tell them it's shit, at least explain to them WHY it's shit. Better yet, explain to them how they can make it not shit. Otherwise you're not voicing a criticism so much as an opinion, and needless to say that never goes over well. Similarly, don't just say a fic is good, say WHY it's good so the writer will know what they did right. Saying "I liked it" might give the writer a warm fuzzy feeling, but ultimately it's no more useful than saying "I'm expected to say something here, but since I'm a lazy ass I'll just try to make them feel good about themselves instead."

Use this form (provided by C.B.) as a guideline. Remember, you can never be too specific.

>I personally like/dislike this story because:

>The objectively good things about this story are:

>The objective problems with this story are:

>The characters of this story are good/bad because:

>The plot of this story is good/bad because:

>The sex scenes in this story are good/bad because:

>I would like to see more/less of:

>I like/dislike the author's writing style because:

>I would personally rate the story, from 1 (unreadable) to 5 (incredible):

And from the aptly named Not Another Shitposter (who has several reviews on his pastebin if you wish to see how to do this yourself):

>How can the author improve? 

The Feedback Sandwich

An effective way of making constructive criticisms is known as the "Feedback Sandwich". It's called that because it's split into 3 parts:
-Start by focusing on the strengths—what you like about the item in question. There is no fic that is completely without redeeming value, so there's no reason why you can't come up with SOMETHING to put here.
- Provide the criticism—things you didn’t like; the areas of improvement. Remember, this has to be something that can actually be changed and addressed by the writer, and provide examples as the writer may not always be aware of the faults in question (perceived or real).
- Round off the feedback with (a) a reiteration of the positive comments you gave at the beginning and (b) the positive results that can be expected if the criticism is acted upon.

When providing criticism, do so within the domain of what you know (as fact) about the person and the subject in question. There’s no need to make any assumptions. Not only does it make the writer look worse than they really are, it also makes you look bad seeing that you will have no possible way of proving or disproving your assumptions on 4chan so there's a good chance you're flat-out wrong. 

Don't Be An Asshole

This may come as a shock to most of us here, but your job as a critic is to help the writers, not deliver put-downs and rant about how bad they are. It might even be the most atrocious thing you've ever read, but just saying that does nobody a favor. If you just want to complain about things and insult people, try going to /a/ or /v/ instead because even if you're the best fapfic writer in the world, nobody's going to listen to you if you act like an insufferable prick. (Yes, this even goes for NiceDaemonette's work. Guy might be a pain in the ass, but at least he's TRYING to write something, which is more than what can be said for most of us.)

"But I don't have the fetish this person is writing about!"

I understand why you might be reluctant to read fics whose kinks are a turnoff. I've been guilty of doing it myself in the past. But just about anyone would prefer critique from someone who admits he's biased and does his best to make critique in spite of that than being completely ignored by people who act on the assumption that any given writer has skin so thin they'd rather have nothing at all. Just give it a shot; at the very least nobody will mind if you focus entirely on spelling and grammar. But if you absolutely can't do it, the least you can do is just say "sorry, but I don't like (kink name here) and I wouldn't be able to give an honest criticism without my bias getting in the way". Literally anything is better for a writer than apathy; if you say nothing, they'll assume nobody wants to read it, which means one less contributor- everybody loses.