Thanks for mentioning Gladwell's book. I have a copy but haven't read it yet. Incidentally, I have a friend who earns a meager living editing books written by some of those $50K Kindle authors.
The highest success rates appear to be in genre fiction, where the appetite for "story" is very strong: Fantasy, Vampire, Horror, Cozy Mysteries, etc. They are often badly written. If they weren't, my friend, whose tastes are on a more literary plane, might starve. But I'll definitely dig out the Tipping Point and give it a try.