My prediction: users will love this, developers will happily trade 30% for better distribution. It is the end of Direct to Fan for app developers.
Back in the old days, if you wanted to sell an Apple app you had to make a really good app, make a really good website, and have John Gruber or Merlin Mann link to it. You waited, hoped people bought the app, and iterated the apps over and over, giving updates out for free. It was a hit-or-miss affair. This is how software sales had always worked, even under Windows and even under most phone OSes – until the iPhone App Store.
Suddenly you had a quasi-curated, easy-to-use, one-click system for downloading apps. It worked really well. This was great for phones. So why not add it to OS X?