The point here is that progressive enhancement is baked in all the way through, eg. using the same URLs regardless of whether a page is being requested by the t

The point here is that progressive enhancement is baked in all the way through, eg. using the same URLs regardless of whether a page is being requested by the thick JS client, an old dumb browser, or search engine crawl. The realtime stuff works regardless of whether websockets are available (Flash & long polling fallbacks). What this means is that you can actually deploy these new features in the real world without freezing out part of your audience, as happened with Gawker's ill-fated redesign. What's more, you can do this without having to develop everything twice, using abstractions like client/server shared language-agnostic templates and realtime libraries like Socket.IO.