As a great philosopher once wrote: don't it always seem to go, that you don't know what you got 'til it's gone? Previews of Mac OS X had a user interface feature, known by all who saw it as the Purple Button. Look at this screenshot from System Preferences:
The boiled sweet on the top-right of the window would go purple, hence the name. Clicking on it activated a single-window mode. All documents except the one that you were working on would be minimised into the Dock, and switching between them would minimise the earlier one before restoring the newly-focused document. Of course, the problem with this in the developer previews/public beta which rendered it unusable were performance-related. The "lickable" eye-candy in Aqua was ambitious even on the top-end G4 systems available at the time, and so time spent in the Genie or Scale effects was really noticable. Add to that the effect of applications being slow enough not to update their views in time - the System Preferences application you can see above is a Cocoa-Java app, and back then the JVM wasn't amazing for performance - and you have a really sucky single-window experience.
On the other hand, it's really bloody useful. Look at apps like WriteRoom or GLTerminal, which go out of their way to get rid of all that other clutter. Or Spaces (or CDE virtual desktops, WindowMaker virtual desktops... you get the idea), also designed to let you forget all those other apps are there. Well, spaces is quite nice (and a little more flexible than purple button was), but playing spaces ping-pong tends to make me a bit seasick. Not to mention the time it wastes being about as great as the unperformant purple button switching...so please, purple button, come back!
Some environments provided the same user experience out of a lack of choice - for instance, OZ couldn't show more than one application if it wanted to, and certainly running more than one at once was out of the question (it would simulate multi-tasking by suspending background tasks).