Both Windows and Mac OS X let you "minimize" windows to the task bar / dock.
Both let you bring a window back by clicking on the task bar / dock.
But on Windows you can click on the task bar icon a second time to minimize the window again. I've got in the habit of using this to take a quick look at a window and then hide it again. I keep trying to do that on the Mac but it doesn't work.
I can see one argument against this feature would be that people often get confused and double-click instead of single-clicking. If implemented naively, a double-click would show and then hide the window immediately, frustrating the user. But Windows solves this problem by treated a double-click the same as a single click.
If anyone knows a way to make this work on the Mac, leave me a comment and I'll owe you one.
One part of this that is nicer on the Mac is that "Hide" minimizes all of an application's windows, and clicking on the dock brings them all back, whereas on Windows it's one window at a time. I have a vague memory that Windows 7 might improve this.