For years (decades?) now, the zoom button has been an annoyance to many people that come from Windows, and has had to be explained and defended by Mac purists, who explain the concept behind it and the ergonomics of its use and how it's superior to the Windows maximize button. Of course, it's never a good sign when a user interface concept, no matter how smart it may seem, has to be explained to users- the whole goal of good interface design is for the user to never have to think about it. But this could partly be explained as a result of a predilection for the maximize function caused by previous experience and not an innate confusion.
Perhaps the more significant cause for the zoom button's death is that developers were just so bad at implementing it. A large number of developers themselves never seemed to get what the zoom button was about, and Apple was seemingly unable to force them to get it, or explain it sufficiently. As a result, it's implementation across the platform was so inconsistent as to be exasperating even for the segment of users that understood what it was for. Basically, you had to remember what it did in which apps, and when pushing it was going to do something useful, and when pushing it would do something unexpected. The end result was, most people, including myself, just stayed away from it all together, except in one or two select apps (Safari and iTunes, in my case).
Anyway, whatever the reasons, it seems Apple has given up on the zoom idea many OS's after its introduction, and repurposed the zoom button as a full screen button- much closer to the maximize function of Windows- although I'm sure us Mac purist will think of key reasons why it is not an imitation, but a completely different, better, idea. :D