- It's the only language present in modern browser.
- It can run on the server, thanks to node.js and the V8 engine.
- It can run embeded in mobile devices via PhoneGap or other alternatives.
- It can be used to create windows 8 applications.
If we're talking about the browser environment, learning how almost every web application should be organized could be one of the paths that you could go on. I'm going down that path, and experimenting with ideas of my own, checking out git repos of popular code organization libraries like dojo, backbone, ember etc.
If we're talking about the node js environment, that's a different story. We're not concerned with minification and file size. Creating entire applications in node, build systems, sites, REST api, there's a lot of things to discover there.
If we're talking about the mobile world, over optimization comes to mind. CSS3 accelerated properties, animations, touch events and trying to use the same architecture as the web with more specialized UI components are great things to start with.
All in all, like others responded, experimenting with your own code, and with the code of others is the best way to get involved, keep up with whats new, and who knows, maybe coming up with the next big thing.