You criticize us!

That's right. Be totally honest and brutal, but do it in a respectful manner. Writing is still a hard endeavor, and no one will ever be perfect. That's why a valid criticism is important. 

Do you see any repeating grammatical/spelling error in the story?

Point it out! It's helpful to let the writer know what they're getting wrong, especially if it's consistent.

I know for me I don't have any beta, period. Which is why I tend to leave plenty of mistakes on my writings even when I try my best to edit afterwards.  It helped me a ton when I had one reviewer on my older story criticize the way I seemed to jump from past to present tense, and it opened my eyes.

Most of us writers here, and on any online writing platforms, are doing this for fun and as a hobby. Meaning our writing skills are always a work in progress. Helping us be better by making us aware of any mistakes, or pointing out flaws in the plot as a whole are important.

What if the writer won't accept the criticism?

Make sure that you presented your criticism in such a way that's respectful to the writer. We all know fanfiction writers don't make a single dime creating new worlds of possibilities. We do this out of pure enjoyment, and to share it with the rest of the fandom. It takes time and effort to write chapter after chapter in a story, and you should bear that in mind as you criticize. 

The point of a good criticism is to help the writer improve. Don't be negative, and don't simply tell the writer that their writing skills need work. Be positive and encouraging, pointing out specific flaws on the story so the writer feels supported and not simply receiving hate. 

Not only criticism, but also praise.

Do point out what you feel writers are doing good on as well! If you feel something especially resonates to you in a story, go ahead and let the writer know! Writers love hearing praise from readers, especially their feelings on a particular scene or development.

It helps to know what we're doing good on so that we know what is favorable in our writing, and continue doing it.

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And that's it! Thanks for reading, and may we all continue learning and improving on our writing.