We've touched on this topic several times already - most recently only a few days ago: the application store model is facing some serious issues at the moment, to the heavy detriment of users and developers alike. If you don't want to take my word for it - and really, you shouldn't, as you should make up your own mind - Marco Arment has written a great summary of all the problems the application store model is facing, with a lot of quotes from other sources to come to a good overview.
Apple's App Store design is a big part of the problem. The dominance and prominence of "top lists" stratifies the top 0.02% so far above everyone else that the entire ecosystem is encouraged to design for a theoretical top-list placement that, by definition, won't happen to 99.98% of them. Top lists reward apps that get people to download them, regardless of quality or long-term use, so that's what most developers optimize for. Profits at the top are so massive that the promise alone attracts vast floods of spam, sleaziness, clones, and ripoffs.
Quality, sustainability, and updates are almost irrelevant to App Store success and usually aren't rewarded as much as we think they should be, and that's mostly the fault of Apple's lazy reliance on top lists instead of more editorial selections and better search.
As the economics get tighter, it becomes much harder to support the lavish treatment that developers have given apps in the past, such as full-time staffs, offices, pixel-perfect custom designs of every screen, frequent free updates, and completely different iPhone and iPad interfaces.
The application store model is under serious pressure.