This is very exciting. The problem of distribution and monetization for desktop software is significant and largely unsolved. Steam is already proof that a digital store can make desktop users really happy, but it addresses gamers, who are cheerful early adopters. Most other people who don't game only know this model through their mobile device, but now they're ready for more. As for developers? For more than half of…
1 - Price deflation. There is a bit of a connotation that App Store means lots of 99 cents apps. Yes, it's not the same platform as the iPhone, but people jumping from the iPhone App Store are going to have some preconceived expectations.
2 - Family licensing. You normally pay a premium for family licensing of a lot of Mac software. This is going to disappear as a pricing tier, so how are developers going to react, given item #1, above?
3 - Marginalization of devs who don't want to participate in the App Store. App Store will be a major way for users to find software. If you're not in there (whether by your choice or Apple's choice), your revenue opportunities could head south.
Having said that, I think the App Store is going to be great for the more non-technical users who use the Mac.