Documents: providing information. News, blog posts, documentation.
Apps: doing things. Email, spreadsheets, games.
On one hand, I completely agree that supporting apps makes the platform complex: browsers are incredibly complicated to build and work on, with such a high ongoing maintenance cost that we only have three rendering engines: Firefox's Gecko, Chrome's Blink, and Safari's Webkit (Blink is a fork of Webkit). On the other, supporting apps is much better than the alternatives.
Outside of a browser, there are essentially two models for getting applications:
Independent installation. You download the program for the manufacturers website, or loaded off a CD. The desktop model.
App store. Your OS has a list of programs that can be installed, and which have gone through some amount of review. The smartphone model.
The web has developed with the principle that it should always be safe to visit a site. As new capabilities have been added this has been critical to maintain. This means you don't need an app store, with power to reject your app.
Mozilla believes that the web can displace proprietary, single-vendor stacks for application development. To make open web technologies a better basis for future applications on mobile and desktop alike, we need to keep pushing the envelope of the web to include—and in places exceed—the capabilities of the competing stacks in question.The web platform has come so far in supporting apps over this decade; we couldn't have made Bucket Brigade without Web Audio or WebRTC. A web developer should be able to do anything native app developers can, making apps for any device, free from vendor veto.
We want to take a bigger step now, and find the gaps that keep web developers from being able to build apps that are—in every way—the equals of native apps built for the iPhone, Android, and WP7.