I've been looking at the Chrome Store and I started to think about developing some nice stuff to publish there, since I have a machine installed with Chromium and I see the WebApp market a lot better than the platform-specific one. But what a site really needs and how it should be designed to be considered a WebApp?
I don't think there's a meaningful distinction. If you read Chrome Web Store docs:
Many installable web apps are hosted apps—normal websites with a bit of extra metadata. You can build and deploy hosted apps exactly as you would build and deploy any web app, using any server-side or client-side technologies you like. The only difference is that you must provide a small manifest file that describes the app.
Therefore even the Google docs sees no distiction between a normal site and a web app.
All a website really needs to be considered the site of a web app is, well, an app. Of course there are many other things that should be on the site such as a Contact page and I believe that a company blog is always a nice touch. I recommend going with a minimalist layout for a web app as no body likes clutter.