So from what I know there is a combo of downloads and reviews, for what I know reviews do not weigh heavily into it. When you launch a podcast on itunes you have a 8 weeks window to be featured on the new and noteworthy section which is like buying an ad on the front page of itunes podcasts. It is cruicial you get as many downloads as possible in the first 24 hours of your launch as possible, that will push to to the top of N&N.