BeOS was (and arguably still is) the only platform with a UI toolkit and system API that strongly encourages pervasive multithreading. Everywhere else, you have the option of writing asyncronous code, but the libraries are all designed primarily for single-threaded use cases.
Multithreaded UI toolkits have been tried and abandoned as impossible to program stable applications in many times, first at Xerox PARC (like so many other things people keep reinventing).
That also made it rather hellish to program, given you had 1990s multithreading technology and debugging techniques to work with. Something more akin to WinRT in Windows 8, where everything works through callbacks on a single thread by default, is a much better way to approach the problem, IMHO.
