I've worked on the build and implementation of mobile-commerce stores for numerous global mobile network operators, as well as brands such as Yahoo and Skype. Some of these mobile storefronts were extremely advanced compared to the very limited functionality of the Apple App Store. Having studied user behaviours, analysed the stats, click-tracked users throughout the storefronts, and spent some months investigating point-of-sale techniques and tricks from the offline retail environments, I am quite surprised at the success of the Apple App Store. There are a number of key differences to this store compared to all the other offerings though:
1. Billing the credit card - clear simple pricing model, and easy payment mechanism, already active for a huge majority of Apple fans, previously using iTunes
2. Only 1 version of each product is required. Stocking a single game in, let's say, AT&T's storefront is much more complex. There are thousands of completely different devices hitting the same store, so you actually need hundreds, possibly thousands of bespoke versions of that game to ensure good handset support. Obviously, you don't want to list the same game thousands of times, so you need clever device detection logic, as well as a smart database to keep track of all the versions of the game. On top of that, you need to constantly backfill for new devices, and render the storefront itself differently for different devices. It's no mean feat. Already Android developers are complaining about 3 versions of the firmware... 3?! try hitting every phone in the market and 3 suddenly seems pretty straightforward.
3. Following from point 2, the user of the Apple App Store is pretty much guaranteed that everything in that store will work on their phone. Users don't have the same feeling of assurance when entering a traditional mobile retail environment (even if it's actually there!)
4. Smooth touchscreen browsing makes a HUGE difference. Most mobile-commerce sites have to focus on the first screenful or two of offered products. Even Apple app store is heavily biased towards the top 5 free, top 5 paid offering, with digging deeper a pain. If you have to click through these lists, then the 'deeper catalogue' is virtually never touched by users - meaning active management of the storefront is required to keep it fresh.
5. Apple users don't buy music in the app store - this is expected via iTunes. Most mobile-commerce sites place music in all the top slots. With the hassle they've been through licensing content from record labels, and ingesting all of that in numerous formats (again to cover lots of device types) - it's only natural that it's given priority over apps. Back to point 4- anything pushed down the page, or onto a second/thrid/fourth page will rarely, if ever be seen by users - so limited number of applications (rather than music or personalisation products) are ever seen.
6. Advertising! Apple advertised what apps could do - as well as the device itself. Hell, to get half the productivity tools/apps etc my Nokia E55 has out of the box on an iPhone would cost me a fortune and many hours scouring the app store. They don;t promote this though.
My feeling is that Apple are very smart when it comes to keeping it simple. One size fits all.