One of the reasons why the Apple app store is so popular is that Apple spends billions on TV/BillBoard/Magazine/Web advertising for the store and apps within it. It's a massive marketing/sales/"packaging" cost.
Which is great if you're one of the top apps and can get Apple's attention, but useless for 99.9% of developers.
I would also say that by far the main reason for the App Store's popularity is simply that it is the only channel for third-party apps on a massively popular platform. Apple could do nothing beyond making it available as an icon on the home screen and it would still be enormously popular.
If you want to say that you're paying for access, that Apple can charge what they like, and that 30% is good value for the money for that access, fine. I disagree, but I respect that argument.
But the person above was comparing Apple's 30% to retail's 50%, which is a bad comparison because retail's cut is largely based on the actual costs of selling.
I would also say that by far the main reason for the App Store's popularity is simply that it is the only channel for third-party apps on a massively popular platform. Apple could do nothing beyond making it available as an icon on the home screen and it would still be enormously popular.