Keep in mind that 50 Shades of Gray was "good" enough to be popular among roughly the same target audience that were already fans of Twilight.
That said, I'm also very curious about why Siduri thinks that HPMoR isn't good enough to be publishable. It's certainly not without its flaws, and I think a professional editor would improve rather than detract from it, but I've found it to be a more fun read than any published book I've read since before it even started being written. A book doesn't need to be flawless to be publishable, it just needs to be able to find an audience willing to buy it.
Unlike 50 Shades of Gray, it's almost certainly utterly unpublishable though, because removing it from the context of the Harry Potter setting would destroy the basis of the plot.
"The kind of classic fifties-era first-contact story that Jonathan Swift might have written, if Jonathan Swift had had a background in game theory."
-- (Hugo nominee) Peter Watts, "In Praise of Baby-Eating"
Three Worlds Collide is a story I wrote to illustrate some points on naturalistic metaethics and diverse other issues of rational conduct. It grew, as such things do, into a small novella. On publication, it proved widely popular and widely criticized. Be warned that the story, as it wrote itself, ended up containing some profanity and PG-13 content.
PDF version here.