Such directories don't do
enough for users, which is why they do not get as much attention as the iPhone App Store.
From the article:
What if Apple did an App Store for Mac software?
I doubt they’d actually do it. Apple loves control and there’s no way that they can have total control over Mac software sales since the current distribution and sales methods are in place and well established.
Apple could release new SDKs for other kinds of machines, which require software distribution through an App Store. This would give them control over such a market.
I think this may well happen. The "Apple Tablet" would be a great platform to get this started. Having such a walled garden would seem great from the non computer-savvy end user's point of view. The user pays an "Apple Tax" and the government polices everything and keeps things clean and safe. As it was and still is (not as much) in the freewheeling "wild west" of the online world, if one doesn't have certain skills, one is a second-class citizen at the mercy of the more skilled individuals. In such a walled garden world, the "law abiding taxpayer" would be the one catered to, and those special skilled individuals become the outlaws. (Or become civil servants and use their skills at the behest of the government.)
For better or for worse, I think that this is actually what a lot of the less sophisticated computer users want! (Witness Linux distros like Xandros.) It's also in Apple's DNA -- getting people to pony up for a well-tended, smoother experience.