writing correct programs is hard, but the addition of concurrency
need not make it harder, and indeed if the problem
includes concurrency (as with networks, file servers and interaction),
having concurrency in the system and language can make it
much easier to get it right, with structurally simpler solutions,
compared to (say) the event driven schemes.

of course, it depends on the approach taken to concurrency.
there are several appealing models, but CSP's processes and channels
model works well.  Communicating Sequential Processes
encourages the composition of sequential processes to solve
concurrent problems.   in its <thread.h> or Newsqueak/Alef/Limbo
guises which all allow sending channels down channels, you can
go a bit further.  unfortunately, many `threads' designs tend by
contrast to be stuck in the lock/sleep/wakeup model of the late 1960s.
that java stuff is a nightmare.