Theres also the question of control...
The inner implementation will implement a thread pool? if yes, with how many threads it will start?
If no, so it will dispatch a new thread for each time?
What about thread affinity and resources that need to assure they run in the same thread it runs before?
The actors pattern is cool and all but in my experience at least they are no the best pattern for every multithreading scenario. So forcing this directly in the language, of course it will make it easy for most people, but as i 've said before if they were built with the current language constructions it would be even better..
Edit: to make it clear.. ? for instance point to optional<> what about an async<> than a sugar could be used of course, but async<> could than be optmized for different cases.. IDK, just an idea..