It also puts you in front of a crowd that is willing to buy your product because that's precisely why they are there.

A mobile web app with no built-in marketplace or buyers takes considerably more hustle to squirrel up a willing audience.

I don't see why this couldn't exist, though. Is this what Google is trying to do with their WebApp Store?

It's not a technical limitation, it's a matter of user expectations and perceived value. In a way it's a marketing issue, that's why Apple was so good at getting over it.

There's also the convenience factor. If I had to whip out my credit card (or deal with Paypal) every time I wanted to buy a $.99 app, I'd buy fewer.

(probably better for me, but definitely worse for developers :)

Yes, but in that case it's just a technical limitation, you could have a unified micropayment platform (wasn't DVD Jon trying to build one? Or something?) which would make it as easy for web services as it is for the appstore.

You could have another unified micropayments platform, but that's still another system I need an account for, etc. (unless you can back-end to iTunes...good luck! :)

The facebook one could takeoff, but then I think you will only be able to offer an app for free and sell virtual goods. Still the same kind of profit margin for facebook as apple though.

Would be lower since they only handle payments, and don't have to manage distribution, infrastructure and a bunch of services around the product itself, not just its sale and billing.