Whenever I use Windows (rarely now it has to be said), I always am struck how little feedback there is. OS X is full of feedback, not just the genie effect, but also actions such as dragging and then cancelling (and it moves back to where you dragged it from, etc). This sort of feedback is few and far between on Windows and so you only tend to notice how much we take it for granted when you play around with Windows. (Biggest pet hate: double clicking an application on the desktop and waiting half an hour while Windows provides no feedback as to whether the program is launching or not, just to be bombared with 200 windows because I got bored and double clicked a few more times 😉 )
On Windows, there is a rather crappy but nevertheless functional minimise effect. Turn it off and it doesn't feel right, even though you know the window will have simply minimised to the taskbar.
I like the genie effect, although I agree that it could be a little slow for some. That's why there's the option to change it to Scale (which provides the all-important feedback, but is also much quicker and less gimmicky).
I once enabled the Suck-in effect but I didn't like it so much. I believe it is disabled because it isn't polished enough. It works well on small windows such as the iChat buddy window but looks completely naff on big windows such as Safari or a Word document (too blocky, as if there were not enough "polygons").
