I'm about four and a half years late to this thread, but what the hell? Here goes.
I participated in NaNoWriMo about six years ago when I was in high school. I reached the 50,000 word quota, but the story was abysmal and absolutely no more work was done on it after November ended. The next two years, I started but did not finish my projects. Since then, I've decided it would be a waste of time.
But this year I think I'm going to do it. I love the sense of purpose and urgency. Even if I every word I write is garbage, even if I fail to complete the challenge, I'm that many steps closer to becoming a successful writer.
Now that that's out of the way - what's the WORST idea you've ever tried to use as your NaNo novel?