The current proposed design requires different await wrapper functions for different types of Promises, Tasks, etc., as well as additional parenthesis, both of which is quite clumsy.
It also requires to bind parameters for direct CPS-style functions, which in turn requires an additional allocation.
It doesn't account for multiple results (from e.g. a CPS-style callback), which also requires an additional allocation plus an additional function call as a workaround.

With direct support for an async keyword or meta and explicit (extensible) compiler-support for arbitrary types, none of these would be an issue.