Distributed shared memory. For large distributed systems these models are hard to implement efficiently and shared memory is not very easy to think about. State space models. Linda and Petri-nets. For large distributed systems these models become hard to implement, and I find them hard to think about.
Interconnection Getting everything so it can talk to everything else is only half the problem.
We also need to be speaking the same language. Trying to find products to do it. Another silly software patent. It violates principles of abstraction and makes a mess of the theoretical underpinnings of type systems. Objects without inheritance are neither here nor there.