For as long as Mac OS X 10.2 (that's when I jumped on board, so I can't comment about the OS 9 days), this has been how Apple has operated.
Mac OS X is actually pretty feature devoid, out-of-the-box. First thing I have to do when I get a new machine is install Google Search Box, Growl, Skitch, MarcoPolo just so it feels like it works right. I don't count these as applications; I think of them as base OS features. By laundry list of features, Windows comes out on top.
What is different is that Mac OS X's UNIX underpinnings, coupled with a thoughtful Cocoa API means that developers can fill in the gaps for power users, whereas everyone else is happy with the experience they get. I'm not sure if Apple's strategists really understand this, or if it is just the engineers. You could be forgiven for thinking the UNIX core is just some accident of history due to NeXT rather than any conscious effort.
iOS doesn't support extension like this, and it's deliberate and calculated. The App Store is not there to protect normal users, it's there to protect Apple. If you gave users the option of App Store apps and also the ability to install unsigned apps (with appropriate scary warnings), all the people Apple claims to be concerned about would function exactly as they do now, downloading from the App Store and avoiding everything else. Why would they need to do otherwise? Instead, we have an App Store that seems to be all about sticking it to Google and maintaining some Disney-esque political landscape. It's nothing like the Apple we used to know.