The first time I heard of National Novel Writing Month, it was while reading a book entitled Beyond the Best Seller. I don't know the name of the author, and can't find even used copies of the book at Amazon.com, but I enjoyed it.
The author mentioned briefly that there is a group of writers who pride themselves on writing the first draft of a novel in a single month. Of course, the author goes on to say, that's not too impressive, being that he knows full-time novelists who have turned out a clean finished book in one frenzied night, simply because they had to.
I think the author, however much I enjoyed his book, completely missed the point of NaNoWriMo, and there are many people (very vocal internet people no less) who have missed the point since.
People who dislike (or mislike, if you've been reading too much A Song of Ice and Fire) NaNoWriMo complain that the novels produced during this time are low quality. They must be low quality, since the only requirement for the contest is to write a certain number of words in a certain time. Theoretically, someone could write the word "moist" (a thoroughly misliked word among female friends of mine) 50,000 times, and win. The bot that checks for victory is basically a word count program, after all.
NaNoWriMo is not about flooding the market with quickly written books, though. Most NaNoWriMo novels will never be sold to a publisher, and that's as it should be, because (you'll know this if you've read as much amateur fiction as I have) most writing is just not that good. Most authors are earlier in the learning process than they think they are, and that's fine. It's the natural order of things.
I think that, whether or not founder Chris Baty knows it, NaNoWriMo is about learning to deal with writer's block. Someone who has successfully written the entire first draft of their book in one month has faced, and defeated, writer's block at least once during that period. Probably several times. Having an "arbitrary guideline" (as the demotivational poster above states) puts you in a place where you cannot give in to writer's block. You have to accept that it can be beaten, or else your story will not be completed.
I find that, at least with myself, the most common motivator to stop writing is fear. Fear that I won't be able to get a scene right, fear that the story is going nowhere, or that I don't have the skill to write it as well as it needs to be written. In my modest experience, there is a fairly simple way to get past these fears.
Just write badly.
Write your scene, or your setting, or your character badly, until you can get to a point you're more comfortable with, and you can always strengthen it when you revise. The knowledge that the first draft can and will be ugly and lopsided and have a funny smell to it can give you a lot of freedom. Freedom to just keep writing, and get down the shape of your conflicts and characters. Later you can bring in the sandpaper and woodstain and make it beautiful on the outside.
NaNoWriMo will also show new authors the wonder that is watching a story unfold before their eyes as they write it. Ending the day's writing in a place that they would have never imagined at the start.
The primary lesson to be learned from participating in NaNoWriMo, I think, is that writing is ten times better than worrying. And participation is ten times better than making snide criticisms from the sidelines.
It's about doing.