I like the idea of a way to make async code more readable but the execution of it in .net is very awkward, every async task is magically run on some threadpool and when the task is done your async function will also continue on that second thread. This means that code above the await and code below it doesn't run on the same thread, and there is no way to control it, this is just asking for trouble if you have other shared state. Shared state and threading is of course always trouble but with the async Task-library you hide it away, with explicit callbacks its easier to see where the entry points of second threads are and you can enclose them in critical sections.

However, in javascript the idea works because javascript is single threaded so you can be sure that the await-"callback" is always run when nothing else is. Explicitly defining some kind of message-pump that you can control the await-continuation yourself would be way around this in multi threaded languages but i haven't found a way to do so in c#.