The thing I like most about the Mac App Store is that it’s both going to help independent developers and users looking for a simple app for their Mac. It’ll be no surprise when the big boys don’t jump into the app store, but it’ll be a big boon for everyone else, even if it’s not as popular as the iOS app store.
The only thing that I think will stop this from becoming a big driver of app sales for the Mac is if Apple imposes too much control over the apps that are available. It is however a double edge sword in some sense because if they let it become a store of junk apps and copycats it’ll become too hard to use and will loose users anyway.