More than a few times, either via a general request I saw or because someone e-mailed me to specifically ask my advice, I've gotten a request to evaluate someone's idea for a story. Sometimes these are too vague for me to help; "Discord and Celestia get into a prank-off" could be a great story, or it could be terrible, and I have no way of knowing from that one-sentence summary. Other times, I have little advice to offer because the concept is seemingly solid. On still other occasions, I've had to point out major potential pitfalls with someone's idea.
I'd like to talk about one specific type of "summary" I've seen more than a few times. The example below is one I made up, but it's representative of both the type of idea and style of presentation I've seen:
Basically, it's the story of Fleet Feather, a pegasus who's in the Royal Guard. He grew up in a foster home and never knew either of his parents, because his father left before he was born and his mother died in childbirth. He's a very good fighter, but not some ridiculous Mary-Sue or anything; he can take most other pegasi in a fight, but he's not as strong as a well-trained earth pony, and he can't wrassle dragons or anything stupid like that. He doesn't say much, and he doesn't have a lot of friends, and in general he's pretty standoffish. He's a good guy, though, and he always stands up for what he believes in. He joined the guard because he was always picked on for being an orphan when he was a foal, and he never wants to feel powerless again. So, he's having an identity crisis because all this stuff like Nightmare Moon and Tirek and Chrysalis has happened since he's joined, and they're way out of his league, so he feels helpless just like when he was little. So, he goes on a quest to find the power he needs so that he'll never have to stand by and wait for someone else to save the day ever again.So, what do you think? Good story idea? Bad story idea? Some flaws, but salvageable?
I'd say "none of the above," myself. In point of fact, I don't think there's a story there at all. There's a character, and (whatever you think of him) it's clear the author has a vision of that character... but a character isn't a story. Even in a true character piece, the character him/herself isn't the story; it's what they do, how they react, even what they think that turns something from "character bio" into "work of fiction."
I see this kind of thinking in a lot of the... let's say, "less-celebrated" stories on FiMFiction, and in fanfiction generally. Stories where it's clear that people want to write about a character (not even an OC, necessarily), but stopped there. When I see a summary like the one above... I don't feel like I'm looking at a summary at all. I'm looking at something that could be in a story, but not even necessarily something that a story can be built around.
Writing a story based on the above would be putting the cart before the horse; instead of having an idea and using or creating a character to communicate that idea, the author is left trying to come up with something coherent to say using a pre-defined character. Rather than picking the right tool for the job, the author's grabbed a hammer and is stuck trying to find a nail to hit it with. Good stories can come to be that way... but my experience as both a writer and a reader has been that the result is more likely to be a directionless, monofocused mess.
So, what do you do if you've come up with a character instead of a story? Hang on to it? You never know when Fleet Feather will come in handy, after all. But rather than try to build a story around him, I try to suggest putting story concept before characters. "Fleet Feather" is not a story by himself. If you have an idea for an epic adventure tale about learning to put your faith in others, maybe he's got a place in it. But nothing good will come of trying to shoehorn him into a anime-eque monster-of-the-week fic. I admit, I don't think much of him, but even a bland cliche deserves better than to be forced to star in a story he was never meant to be a part of.