One of the big things about NaNoWriMo is dealing with time. It seems as though evenings after work are both incredibly short and incredibly long when you know you have to write 1700+ words before you go to sleep. It's weird.
Also, I'm at around 35,000 words right now, and I still don't know if I'm going to run out of stuff early, or stall too long and have trouble wrapping it up in time. I'm not precisely sure how it's all going to work out, even now. (Think it's suspenseful for a reader? It's even worse from over here in the author's seat.)