I've kind of been lurking on the @ao3commentoftheday discussion about concrit lately, and I wanted to jump in with my two cents.
1. Not everyone who writes fanfic cares about improving their writing.
This is something that a lot of people defending concrit seem to be missing. THEY want to improve, and so concrit is important to them. Concrit helped them along the way. And that's great!
However, some people aren't trying to improve their writing. For some people, fanfic is just a way to de-stress, and they're perfectly happy to share things with whoever enjoys them, but they aren't interested in improving it.
That's okay. That's valid.
We share fanfic for others, but fandom is first and foremost a personal thing. You enjoy it in the way that suits your wants and needs.
Like with anything else, it's a hobby for some people, and a passion for others. If it's your passion, great; but don't demand that someone's hobby become their passion, just because it happens to be yours.
2. Not everyone wants concrit from the whole world.
Even people who want to improve often have a few trusted friends they welcome serious criticism from. Now, I mean truly trusted; friends that they trust to tell them when their writing is OOC, their plot doesn't make sense, or a line of dialogue is choppy.
That doesn't necessarily mean they want concrit from strangers.
Criticism that comes from a trusted relationship is often more appreciated than a stranger.
3. If you're excited about concrit, great! Find a group of like-minded writers, and help each other improve! They will appreciate it, just as you will appreciate them.
1. Some people want concrit.
3. Some people want concrit from specific people only.
Regardless of your personal feelings on it, please respect the boundaries of others.
It doesn't matter if your advice could turn them into a best-selling author. If they didn't invite concrit, don't force it.