The worst is COM's apartment threading model, which is still in your face to this day despite being pretty much nothing more than an extremely elaborate workaround for the fact that Visual Basic 6 didn't support threads. (VB6 was released in 1998; it became old enough to drink this year.)
I doubt that VB6 is actually the only justification for COM's apartment threading model. If that were the case, then I don't think WinRT would still have ASTA (application single-threaded apartments). I think the real rationale is that making sure certain code runs on the UI thread is something that applications have to do all the time, even today, even on non-Windows platforms.
