Slightly off topic, but I've been working in Haskell, and recently realized how the laziness of the language means everything is basically asynchronous by default, but you can still write your code in a synchronous fashion. No real need for promises or callbacks because lazy values are the promises. Well, when using lazy IO, at least.

I'd actually much prefer lazy values be put into Javscript than almost any other feature (real module support is another big one).