The basic architecture is browser <-> web server <-> database.

When the web browser connects to the web server, the web server opens a database feed. When the database pushes changes to the web server, the web server pushes them to the browser via socket.io.

Is this not already possible with MongoDB, using something like mongooses post('save') [1]?

[1] http://mongoosejs.com/docs/middleware.html

You could do it with mongoose middleware, but this solution is a lot more complex and expensive. In this case the application developer would have to take care to notify an additional piece of infrastructure about every change. Even if you can abstract the code, that requires quite a bit of additional intelligence on the backend.

Baking feeds into the database should dramatically simplify the amount of required work.

I use WebSockets. You could also use Server Sent Events.