The unambiguous ⇩ ⇧ arrows for minimize/maximize. Instantly recognizable pallete icons. Straightforward titlebar without pixel-wasting clutter. With a contemporary color scheme there'd be nothing wrong with it today - sure is more intuitive.
I get that they probably (I'm assuming) represent moving the titlebar to the top (maximizing) or to the bottom (minimizing) of the screen, but I can see someone who doesn't already know what they do assuming they're for changing between windows.
To be fair, the modern maximize/minimize icons that Windows uses are not that much better. The minimize button doesn't even represent what a taskbar entry looks like (by default) anymore.
