Hi, @Desiderius Price and all.
To the all the above, I would add, “First, read the story.” There are some readers “out there,” in multiple meanings of the phrase, who will pan a story over points that were covered in the story with crystal clarity had they only bothered to read it.
Otherwise, @WarrenTheConey’s key points are a very good start. Keeping in mind that beta readers are scarcer than hen’s teeth, most fanfiction writers are having to figure out everything all on their own. Any advice sincerely and honestly given will help the writer in posting a better story.
The “great story, keep posting” reviews are also nice, especially for longer stories. While I don’t write for reviews, I also don’t turn my nose up at good reviews that only tell me that the reader is enjoying the story.
Cheers!