Go doesn't "force" you to use one method of concurrency.

It has more than one (you can do erlang-style share-nothing style or Java/C++ style of using mutexes to protect shared state from concurrent access).

I know nothing about Closure so it's possible it has more features but it's not necessarily a good thing. Is the complexity of 4 different solutions worth it? (by "it" I mean: a programmer has to learn all of them and when to use what; the implementor has to implement them; write wrappers for all standard data structures (what about third party libraries?) etc.).

Feature bloat has a cost.

Go gives you all you need to easily write concurrent programs and it does it with refreshingly simple design (both for people to learn and to implement).