Apropos of yet another round of exasperation with commenters who somehow think they're fit to be my beta or that it's okay to put a review in the comments:

Let us remember, please, that comments on fic are a direct, one-on-one conversation with the author, in the author's space.

If, during a party, some dishes were served that you happened not to like, or a dish was prepared in a way you found odd, you would not go find the host and/or cook and inform them that you suppose they made a good effort but that lamb had too much cardamom in the sauce. Not unless you were an asshole, at any rate. A good friend might sneak into the kitchen for a taste beforehand and note that maybe the sauce could use some alterations, but once it's done and on the table? If you don't like it, you just don't eat any more, possibly mentioning to another guest you know shares your taste that they should stick with the olive spread. This is a party, not a restaurant. You didn't pay for this food. You are the beneficiary of the host/cook's generosity and a guest in their space, and there's nothing to be served by telling them they didn't make the lamb the way you, personally, like it. You won't get lamb the way you like it, next time; you'll probably just not be invited back, since you clearly have such a huge problem with the host's lamb recipe that you just had to tell them about it.

Fic doesn't even carry the potential pressure to stay that a party does. There's no shadow of an excuse to buttonhole the author and complain over a fic not being written to your personal preferences.

Reviews are different. Those are in the reviewers' space (or should be, thank you very much) and those are for the benefit of readers. Pointing out both what you liked and didn't like is not only excusable but, indeed, the very business of a reviewer. An author who demands only positive reviews is delusional and living in diva-land. An author who demands only positive comments to the fic itself, and that reviews be addressed to their actual audience not to their topic, is telling readers to remember whose "house" they're in and who they're speaking to.

Besides, what do you think will happen, if you do comment that way? Will the author go and revise that finished story? No. Will the author magically start writing just the way you want them to? No, not even the ones who think they should try.

But doesn't the author want to improve in her art? asks the devotee of the Holy Concrit, puzzled and offended that their faith offering has been rejected.

Some fandom-states subscribe to the Church of the Holy Concrit as their official faith, and accept or even encourage all readers leaving "how to improve" comments. Some most signally do not. Anyone tempted to proselytize their personal faith in heathen lands might find it instructional to look up the Crusades (One through Nine) and just how little result they had outside of vast death and destruction. In little, that's pretty much the pattern you'll see over Concrit, too.

Teal deer: in someone else's fandom space, check the profile/info/about before you assume it's hunky dory to tell them to their face all the things you, in your towering judgement, didn't like about their fic. Failure to do this may result in a swift smack on the nose.

Didn't like someone telling you your comment was substandard? Remember that feeling.