And it's not that I think Celery is the best. It has huge drawbacks.
I merely suggesting that the model Celery and GCD enforce is much better for writing CPU, I/O and CPU+I/O bound code.
Well, I need a system that is responsible for maintaining the shared data, and I have to program my tasks to send it queries and updates, and receive the results, and so probably I'm going to need serializers and deserializers for all my data structures, and if the tasks have transactions (which they almost certainly do) then I'm going to have to implement locks to maintain the data integrity. Whatever it is, this system looks a lot like a database.
Whereas with coroutines each task just updates ordinary data structures in memory. No need for communication, queries, serialization, or locks.
Queue and Thread is available in python and work well up to a point. Twisted (and sugar @inlineCallbacks) or similar makes for much more concise code.