Over the four years I've been reading and reviewing fanfiction, I tend to only leave positive comments (or no comments at all). I don't feel that folks post to Stories of Arda, etc. to get grammar or plotline advice, but to share something they're excited about. If I read a story and all I can think of is, "Please learn to use commas!" or, "There's nothing of the author here, she's just regurgitating Tolkien almost word for word", I usually don't review at all.

However, if folks post somewhere (like the community you're suggesting) and truly are asking and hoping for honest advice, I'd try to leave honest and helpful with comments.

I'd certainly be interested in seeing where this goes.