The cool thing about the Internet OS with web apps as features is that the features are interoperable. I post a picture to Instagram and push it to Twitter and it comes through as a picture (or it did until the kids started fighting). I find something I want to buy on Etsy and I check out on Paypal. You get the idea. Web apps pass data and users back and forth easily.
That is not true on mobile today. Even on Android where sharing is baked into the OS. It is a lot worse on iOS. But its bad all over the place.
If you post an Etsy item to Facebook and I want to buy it, I click on the link in Facrbook mobile and am taken to Etsy's mobile web app where I am not logged in and its a pain to buy. I want to go app to app to Etsy's mobile app where I am logged in and its one click to buy.This is a very important problem and something that I've touched on in previous posts on this blog.
Doing app-to-app handshakes is doable today: Android's Intents model was baked into the OS from the very beginning and supports them quite elegantly. On iOS, you have to mess around with URL types and parameters, but that isn't rocket science.
I expect that app publishers will start paying a lot more attention to making their apps interoperable. This will likely start with e-commerce apps. When you're actually trying to buy something, as in Fred Wilson's example, real dollars are at stake, and deploying the engineering resources to build interoperability makes economic sense.