Ah. Again.. they've implemented that in the terminal but not much else.
It's something that would be nice but certainly not a dealbreaker for me personally. Certain things follow the mouse (scrolling, for example. I can scroll the window under my mouse even though it's not selected). I can see a number of stumbling blocks for this on the mac (mainly owing to the pervasive application menu) but I think they could all be overcome with a bit of design savvy.
I'd be happy to see the feature but I don't need it.
As a long time Linux and Mac OS user (80% of my work time is spent on a linux workstation) I can unequivocally state that for my needs any downside to OS X is far outweighed by the upside. I'm not sure about other desktop environments (nor do I really have the time or energy to try any to be honest) but I would much rather use my Mac than spend time in KDE. I get all of the unix goodness that I love and I get an absolutely wonderful GUI to boot.