Stories and Tools - Anil Dash
This post is ten years old, but I think it might still be the best attempt to demarcate a difference between web “sites” and web “apps”: think of them as stories and tools.
It’s also remarkably prescient about the need for an effort exactly like HTML5:
A widely-distributed, standards-compliant, browser and platform-independent library of functions that would perform the basic user interface functions for a web-based tool, relying on the server side only for the logic and data sourcing.