Dance With Them What Brung You
The Dance with Them What Brung You School of Criticism is a fairly
simple principle, and it applies both to betaing and reading for
pleasure: the story is what it is.
Let's take a totally made-up story for example... we'll call our story Jonathan's New Dress. This is a gripping tale of Jonathan Kent's secret life as a crossdresser. Now, there will be opinions on all sides as to whether or not Jonathan would ever enjoy the way that lace and silk panties make him feel sexy, and that, in itself, is a fair topic of debate.
If you volunteer to beta and you personally feel that Mr. Kent is a Hanes tighty-whities kind of guy, always and forever, you have two choices: tell the author you don't agree with the characterization and gracefully suggest they pass the story on to someone else more likeminded to complete the beta, or... dance with them what brung you. In other words, help the author make it the best damned Jonathan-in-a-dress story that's ever been written, regardless of how you feel about the genre.
Likewise for readers: if you know you hate reading Jonathan-in-a-dress stories, don't read it. (Or rather, let's cater to human nature, go ahead and read it, snicker to your best friends about it, but leave the author alone.) What's the point? You know you don't like it, no one's going to change your mind about it, and thwapping an author upside the head for writing a genre you don't like reeks of self-entitlement. That's not to say discussing the genre in and of itself should be off limits, far from it (just try to keep me away next time rape-as-romance comes up in the fandom,) but on the individual level of the story, if it's something you just don't like, that's your problem.
On the other hand, if you think the author could have made it more convincing in X way or Y way, then by all means say something. If you think the author failed in Z or A fashion, say so. Was the story internally inconsistent? Inconsistent with the canon? Did a plot point fail to land its mark? Even down to characterization points, (In the episode "Silky," Jonathan said he prefers flannel. I find it hard to believe he'd change his mind for panties...) Grammar, spelling, it's all fair game. Point being, you dance with them what brung you- the story is what it is, and you address it on that level. You can't reasonably fault a Jonathan-in-a-dress story for not being a Lex-in-a-dress story, or a Jonathan-on-a-tractor story, etc..
Dance with them what brung you. Stories won't always have the ending you like, they won't always have content you like, they won't always play into your personal desires and preferences for Jonathan Kent in boxer briefs. And there will be complaints that some authors didn't warn you about the shocking appearance of full, knee-length boxers, but it's my opinion that every story has a warning. Those with explicit statements are easy to pick out; those without any warnings should be considered stories that may contain your every worst bugaboo- proceed accordingly. However, there's a difference between pure preference and actual error.
I believe in public criticism and critique of stories posted in the public- I think it's valuable for the fandom, for the readers, and if the author is thick-skinned enough, valuable for her, too. But I think there is a vast difference between "This story didn't hit my happy buttons," and "This story had these flaws." Unlike professional criticism, nobody's making us read fan fiction. Savaging Gigli is cathartic for somebody who had no choice but to sit through it; savaging Mary Sue Fan Fic Author for writing a story in a genre you don't like is just plain mean. Forget what your mama told you- no need to be nice, but if you don't have anything useful to say, don't say anything. You don't shame a peach for failing to taste like an orange.
And you dance with them what brung you.