One of the most significant changes in Windows 8.1 relates directly to the Windows Store but doesn’t involve any new APIs. This is the auto-update feature for app, which means that you no longer need to rely on users manually updating your apps from the Store. The primary impact of this is that if you make feature updates and especially add in-app purchases, those features will become available to all your customers unless they’ve specifically opted out of auto-updates. This should help quite a bit with monetization over time.

At //build 2013, Ted Dworkin’s talk gives the overview: Windows Store Overview for Windows 8.1: New Design, New Promotion and Monetization Opportunities (2-123).

As for new APIs, the key features here are added support for consumables (in-app purchases that can be re-purchased over and over), and support for large catalogs of in-app purchases (going beyond the 100 item limit of Windows 8). See the Windows.ApplicationModel.Store APIs, as well as the talk Monetization Opportunities for WIndows Store Apps in Windows 8.1 (3-121).

Speaking of the Store, there are a couple of other related talks from //build 2013: