fanbois mode active (sorry)
The app store idea adds a lot of value for "normal" consumers:
1. apple checks things for malware, viruses, backdoors, etc. Whereas anything you download from the internet at large has always been a crapshoot. Apple has been lucky that malware usually targets windows, but that won't last forever and I think they want to change people's habits before it's too late. The walled garden keeps you in, but it can also keep bad guys out.
2. Apple checks that the app at least does what it says on the tin and has a rating system to keep sellers from being dishonest or misleading.
3. It's a single place where you can compare apps and make selections. You have all the choices in front of you at once and you can make a decision (on features, price, screenshots, ratings) without wondering if you're missing something. (Admittedly, that's only a feeling. In fact you are missing out on choosing from non-mac-store apps.)
4. iOS apps are dirt cheap, often free, so you can try out a lot of stuff with no financial risk. if you decide an app is important enough to pay for a nifty feature or to get screen real estate back from the ads, you can make that choice after you've had a chance to try out the cheaper/free stuff first.
One hopes that apple will go father:
A. It would be awesome of mac apps adhered well to apple's HIG. The minimalism of iOS apps is refreshing and one hopes that mac store apps will find a nice middle ground of maximum utility with the minimum of buttons, tabs, menus, pop-ups, etc.
B. Non-lame games. iOS app store is game-console-like in that, with only a few exceptions, any app can run on any iPhone or iPad. They have made one division of the space for iPad-only devices and that's it. But I think the mac store should be more fine-grained and have an idea of your mac's performance and steer you toward apps & games which guaranteed not to stutter or have miserable frame-rates. (The devs have an interest here also: they don't want user hardware problems dragging down their ratings.)
C. Repeating the middle ground thing vis a vis file systems, there is a huge disconnect between the number of files exposed to users on the mac (thousands upon thousands) and the iOS approach of having almost no files at all visible. I'm not sure what the answer is, but there should be some concept of accessing the same data from different apps, but without making the user navigate a hierarchy or see all the irrelevant files along the way.