I believe that BeOS (and now Haiku) explored some ideas like that. I think that all (most?) OS APIs were async and message queues were used to call the APIs as well as return values from them (someone correct me if I'm wrong). This resulted in a really snappy GUI but that's about all I know about it. One major hurdle was that programming for the OS became tricky.

There's this free book about BeOS which I've been meaning to read http://oreilly.com/openbook/beosprog/book/ but never quite got around to it. You might find answers to some of your questions there.