I think that's all you need for version 1.0. All the bleeding-edge concurrency abstractions like software transactional memory are only that: abstractions. Henning is right that they're going to become increasingly important in the future, but as long as you have threads and atomicity as building blocks, you should be able to do all the rest in the macro system.

Just make sure that some concurrency package eventually becomes a de facto standard. Getting C libraries that use different threading implementations to interoperate is a nightmare, and I'd hate to see Arc go the same route.