I have only to look at the word counts racked up by people in my various writing groups to know why I don't participate in Nano. I'd be the horse that doesn't make it out the starting box!
On a good day, I'm happy to clock up 1.5k words, most days I'm delighted with 2 pages. I love the idea of completing a first draft in a month, probably because I love editing so much and there'd be so many juicy bits to dig into. Which is the root of my problem.
I can't skip scenes and my story staggers to a halt if there's something in an earlier chapter bothering me. I re-read from the start at least once a week, editing as I go before continuing to the new stuff. And if I'm writing a historical, I take long pauses in between for research. This used to drive me nuts and I tried to wean myself onto better, more productive habits, but I finally realised that creative streams are very much like the proverbial river, you don't get far when swimming against the natural flow.
On the brighter side, once I'm finally done, I really am done and there's not much left to edit--at least until my editor gets hold of it!
Happy Nano to everyone out there, I might not be particpating, but I'm cheering from the sidelines and swallowing down my awe.