Good points NoHaikuForMe. In the end nothing is a panacea for bad programmers or bad assumptions (your file existence check example is a good one.)
But you leave out the fact that having multiple threads per application leaves less chance of locking up the whole application. Even if one window is stuck checking for that file on a CD, another window would still be usable. And the application itself can still process messages. Of course as I’ve said before this all comes at the cost of more programming complexity.
As for continuation passing style, I don’t really know enough to comment on implementing that in Haiku, but if continuations were truly useful they would be used a lot more. The only relatively “mainstream” use of them that I know of is for Seaside, a Smalltalk web application framework. Ruby has them but I’ve only seen them used for tricks and clever code examples. If people have trouble understanding threads, they will really have trouble understanding continuations. Except maybe for Lisp or Scheme programmers and they are rare these days.
Also I do not think it would be very easy to add support for continuation passing style to other operating systems either, so I don’t count that as much of a point against Haiku or BeOS.