As I teach my 200 level English Composition class, it comes up time and time again. Writing is a skill. Just like anything you do, it requires practice.
As someone who is a writer, I take these things for granted. I take for granted the ease with which I develop ideas, or imagine, or finesse. I take it for granted because it is something I have practiced so often and so long, I no longer think of it as just a skill. I can't really remember learning how to write. I just always did it.
It strikes me how important the practice was. I distinctly remember making illustrated books in elementary school. I even recall several characters I dreamed up. Obviously the years of writing between then and now has made quite a difference. I wouldn't be caught dead publishing what I wrote then. But it was a start. It forced me to think about these things and experiment.
The reason why NaNoWriMo is such a good thing is because it forces people to start and practice writing. And having a reason to practice with other people such a solitary activity is huge. With that, I'm going to go practice.