The first problem I have with NaNoWriMo actually only comes into my mind about a month later when I finally get around to trying to write some more on the novel. This problem?
The fact that my book sucks.
In fact, the problem is that since I give myself the ability to write badly in November, I do write badly. Which is the point of a rough draft, of course. But then when I come back to my book in December, I'm suddenly struck with how completely unrealistic and horrible my book is.