From building http://platform.qbix.com/features/streams I know that there are a lot of challenges here. Caching, throttling and batching requests is of course really useful. My biggest question is how do you implement:

  * multi-user access
  * realtime updates to connected clients, and 
  * offline editing and sync?

When multiple users access a resource, you have to consider things like consistency issues (making sure all users see the same changes applied in the same order). Also other stuff would be nice like access control (perhaps integrated with the publishing user's address book) as well as subscriptions notifications when something changes.

Does anyone know of a library that implements that? It took me a couple years to get everything right in my own library.