The mac will NOT be app-store only. However, if you CHOOSE to run app store only apps, you get sandboxed, vetted apps from a trusted vendor. Windows 8 is going the same way
Metro applications in Windows 8 will only be available through whatever they call the Windows App Store.
Software developers are mainly concerned with creating software that works, and that works well. "UI designers", on the other hand, are more interested in creating software that looks "pretty", even if it's damn impossible to use productively.
And that's why we need the UX people.
that allows users to purchase a product from another source without leaving the application... the product source, which is different than the application source... gives the impression to the user that they are purchasing the product directly from the application
With Steam you buy it from Steam through Steam. With Apple's in-app purchase you buy it from iTunes through the third-party app.