I haven't read Gladwell's original book on this, so I don't want to criticize him directly, but I think the 10 000 hour rule, as I've heard it summarized, is over-simplistic. You need to practice to get good at things, sure. But you also need to have some basic aptitude.
I could practice the skills required of an NFL linebacker for 10 000 hours and get much better at them, maybe even to the point that I could be considered an expert, but I still wouldn't make the NFL. I'm 5'8" tall and weigh 140 pounds. Similarly, I could put in my 10 000 hours of practice at being a gymnast and I still wouldn't make the Olympics - again, I'm 5'8" and 140 pounds!
I think most people accept this in regard to physical accomplishments, but many people seem to resist it when they look at intellectual or artistic achievements. In my opinion, though, it's equally true in both areas. Putting in 10 000 hours of conscientious, directed practice writing will absolutely make you a better writer, but it may not make you a good enough writer to achieve your goals (of publication, or writing the great American novel, or whatever else).
I wonder what the totals would look like if we compared the number of people making a living from professional sports to the number making a living from professional writing. I wouldn't be surprised if they were roughly equivalent. Most people agree that one needs to be physically gifted AND to work hard in order to excel at sports; I would argue that one needs to be mentally gifted to succeed at writing. Practice is necessary, but not sufficient.