[moe]

If you're writing any kind of significantly complicated asynchronous process, what is the difference between following a bunch of Deferreds and following a bunch of callbacks??

The difference is that in co-routine based code (such as concurrence or eventlet) there are no callbacks. Co-routines are all about message passing which leads to much cleaner and much less code in most applications.

It's not uncommon to see 40%+ boilerplate callback-handling code in a twisted app. Those are not needed in a coro-environment.

Asynchronous coding is hard.

Actually the established patterns for implementing asynchronous code are hard. You get to choose between threading-hell (deadlocks, races) and callback-hell (code-bloat). I have no idea why these abstractions grew so popular, my guess would be because they are "closest to the metal".

But they are not without alternatives; co-routines, actor-based concurrency and erlang exist - all of which enable fairly straightforward "Do what I mean" development.