Tornado made async code slightly less painful by using yield and coroutines, but you still have to run blocking methods on thread pools using futures. They abstracted it really nicely and I can now write clean code if I need an occasional blocking library in my tornado code.

But after writing tons of Go over the past 2-3 years, going back to async code, even with the tornado sugar, just feels like driving a manual car after getting used to automatic. It's just redundant. I've seen better, I've written way cleaner code and got better concurrency. Promises, futures, yielded generators - they are all syntactic hacks. The only language I've used that really addresses this properly is Go (disclaimer: I haven't written any Erlang).