I was happy to fall upon this nice explanation by Steven An, left in a comment on a Gamasutra article about the new Mac App Store:
I think the primary thing that Apple did was create and market the concept of the "app" as a $1-5 unit. They're doing to software what they did to music: they broke it up into little pieces and then gave consumers a nice place to shop for the pieces. Before, the channels of distribution were much clunkier and inert. This is very much a good thing for smaller devs. Apple has removed so much of the headache of marketing ("Just find it on the AppStore!"), setting up payment systems, installation, etc.
This is a good start, but it only describes the business logic of the app store and its resulting apps. There's a bigger question embedded in An's observation: what happens when we break software up into tiny pieces?
