The App Store will likely work and big companies will place their software on it (but I don't think Apple will accept them, the terms look very restrictive). The big comapnies will just raise their price as way around giving a Apple 30% cut. Funny thing is, since its Apple-related and many people are not computer-savvy, there will be plenty of people who will pay these inflated prices.
I just have a feeling that the Mac App Store is going to dilute the quality of Mac applications. We're gonna plenty of crap fill it up, like clocks, calculators, and period calendars.
And how is Apple going to stop people from pirating their Mac Apps (by using an Installous-type application). Right now, the vast majority of people don't use Installous because you need a JB iPhone/iPod/iPad and the vast majority of iDevices are not JB. However, with a Mac, it's already "jailbroken", so what's to stop people from developing an Installous-type and having the masses use it? Morality?