I came across the following quote about threads while reading The Evolution of Lua:

… we did not (and still do not) believe in the standard multithreading model, which is preemptive concurrency with shared memory: we still think that no one can write correct programs in a language where ‘a=a+1’ is not deterministic.

That is quite possibly the best one-line summary I have read about why shared-state concurrency is evil. Just say no!