I think that the 70/30 revenue ratio for author/apple in the Mac Store is a huge down side. While the Mac App store certainly may bring newfound attention to an application which previously would not be as known to the public, Apple's goal is to get more Mac marketshare, and more profit from that. While the App store may give Apple profit from leeching off developer's apps, no other operating system has that. It will be seen as a downside for developers to develop on the Mac, and that is the opposite of what Apple wants. With more developers for the Mac, there will be more users and more attention, and ultimately more innovation and leading progress - my speculation is that this will discourage developers to develop for the Mac by not getting all of the profits.
Also, as a 16 year old who only has $50 on my PayPal currently (and that happens less than once a year), I'm going to be weary on what applications to purchase and which to save my money on and pirate. While this may be objectable to many, I try my best to purchase my music and applications I use, and once I get a credit card I plan on doing so much more efficiently. The reason I might purchase an app is to support the developer and the firm. When Apple gets 30%, which is a big chunk of my money, it is discouraging not only for the developer but for the end user - especially a user like me with limited amounts of money - to purchase anything, because I know that the money is not going to who it deserves to be sent to.