At the risk of giving away a million dollar idea; nobody's gonna wanna really edit things in the browser. Too risky and missing features of whatever their favorite editor is.
But how about letting people push their code up via git? So you could have a runnable browser window open, push your changes up to your runnable repo and then hit run?
If this does turn out to be the key that breaks this open please remember it was my idea.
Here's another idea. JSBin is doing something pretty awesome: it uses the HTML5 File API so that you can select a file from your computer and it syncs to different instances of the same bin while you edit it in your favorite editor. Check it out: http://www.youtube.com/watch?v=mY56fNmn2cE (keep in mind the video was taken during development, now it's pretty stable).
BTW some people like editing in the browser and that's why Cloud9IDE is alive.
Soooort of off-topic, but I've actually had a lot of success using Cloud9 IDE (https://c9.io/) as an in-browser editor. It really closely mirrors Sublime Text 2 as far as default functionality and appearance goes. Of course, I'm coming from a Visual Studio IDE background rather than a Vim/Emacs background, so I can't vouch for its usefulness for users more accustomed to that type of work environment.
Not really, webshell looks like there are trying to become an All-in-one API of APIs. Runnable is trying to be a platform for sharing code examples for APIs, modules, and any other full-stack code snippets.
Looks like Webshell is integrating APIs into your site, while Runnable is about API discovery, prototyping, learning from others (more of a Stack Overflow direction).
Here's what I've been looking for: let me put a badge on my open source project on GitHub which launches a Runnable with the project so that people can play with it without having to pull the source and set it up themselves.
But how about letting people push their code up via git? So you could have a runnable browser window open, push your changes up to your runnable repo and then hit run?
If this does turn out to be the key that breaks this open please remember it was my idea.