I think it has to be observables, I was pleasantly surprised when I came to know about them (through MobX) when I tried them out.

It is already quite popular, but the concept of React Native (and React) to learn once, and write everywhere; with same concept of components, but reconciled to different things (DOM in the Web, Views in the Mobile) should also get more exposure. You would be surprised at how many people still think that React Native is just a fancy wrapper over the web view.

I only recently became aware of Electron. To me, it's very interesting to box in an entire app written in JavaScript to run on the desktop instead of within a browser.