I did something like this in my presentation[1] about Ember.
It uses Ember.js to maintain the state of the code samples and proved to be very stable. I don't have interactive sliders on specific values, but the code is parsed and run after every character the user types. e.g. you can change the size of the squid on this page[2] and see it reflected immediately on the right.
I've got a few friends who are affected by this who haven't tweeted in months. All posts come from the web interface. Can't find any news or security sites with more info.
The source code is a treasure trove for those looking for an example of a larger Backbone.js application with a Rails backend and Faye for push notifications to the browser.
I built http://gitego.com a few months back to keep track of watchers, forks, size, and issues over time. I'm glad they're enhancing the code-related graphs. Stats for those would be much harder to gather externally.
I run a website that tracks watchers on GitHub projects, http://gitego.com.
What I see is that it is incredibly rare for a project to lose watchers. It seems that people are using GitHub's watch feature as a bookmarking service.
On a side note, GitEgo tracks two[1,2] of paulasmuth's GitHub repositories, and neither of them has "lost lost around 30 watchers by the evening." He may be referring to other projects on GitHub, but even Twitter Bootstrap[3] (the most popular GitHub project), almost never loses watchers. Granted, these projects could all be gaining just a few more watchers than they are losing per hour, but the net effect is almost always a gain each day.
I think the real problem is that watching a repository doesn't engage a user in its contents. Watching commit messages fly by isn't as entertaining as reading 140 characters someone groomed for public consumption.