I've always been more of a believer in quantity over quality. I'd rather have one nice thick fuzzy sweater than three thin things that can barely keep me warm. The same is true for writing--I at least have to like what it is I'm writing.
That being said, doing NaNoWriMo is opening me up to the merits of brute force writing. Screw pretty, screw prose, screw grammar, screw spelling--just write. Eventually you'll get an idea at some point (usually the 10 minute mark). And eventually you'll reach your requisite 1667 words for that day.
Having this requisite number changes the way I write: it's liberating, in the sense that I'm not as worried about structuring the story and can just focus on telling it--although that's probably due to my extensive outline and pre-NaNoWriMo work. I am not worried about tangents, not worried about developing themes (I am going to have a headache and a half when I go back and develop all of the themes which have popped up so far), not worried about character development. I am just writing 1,667 words per day, every day.