As a developer you should always be looking at other languages and striving to be a polyglot programmer. Learning a second language will make you a better developer in your primary language. This seems like a strange statement but what happens is that you see a strange construct or pattern being applied in the second language you say "hey, that's not available in my primary language." On further investigation, something like that or a clever workaround is usually available and that then gets added to your arsenal of tricks in your primary language.
2. Programming styles
3. Full stack
With Node.js it is now possible to use one language to develop web applications on both the client and the server. If there's one language to rule them all then this is it.
There is no other language that will run in all browser and that has a committee defining and developing standards for it.
Okay, I just made this one up. However, it might be true. I have no idea how you would measure this but I bet that if it's not true today it will be one day soon.