I agree, async is much more flexible and powerful when it's a library feature (as it is in F#) as opposed to being a built-in language feature (as it is in C# 5

I agree, async is much more flexible and powerful when it's a library feature (as it is in F#) as opposed to being a built-in language feature (as it is in C# 5). core.async looks very well-rounded, congratulations on a fine job.