First off I've never used either BeOS or Haiku (I can't run A2, will try latest nightly this weekend), I just became really interested in Haiku after hearing all the good things about BeOS and the aforementioned. From what I can decipher as an outsider, though, BeOS had double clicking to run apps/open files and folders from the desktop and Tracker, just like Windows and (I assume) OSX. I'm a long experienced Windows user, so double clicking doesn't bother me much. Then I read this article: http://www.codinghorror.com/blog/2004/10/double-click-must-die.html
This is pretty spot on, now that I think about it. Double clicking is NOT exactly a user friendly (or developer friendly) function, my parents can attest to that, they still don't seem to understand when to double click or click once. And when you think about it, it's not really their fault. The rule for when to double click or not, is very obscure for newbies, and for some things (such as the system tray) even experienced users can't know which tray icons use double/single click functionality. The act of double clicking itself is VERY unintuitive as well, I remember how awkward it was for me when I started using Windows as a little kid.
My point is, for the sake of user friendlyness and logical GUI design, we should remove double clicking, at least as a default? Maybe even make use of more than 2 mouse buttons so selecting and launching an icon aren't confused, under a single click design?