I'm talking about the kind of zoom that allows you to start in the thick of the action, camera on the ground, and with a scroll of the mouse wheel you're transported up an away until you can see the entire map at once.
Then, with a second spin of the mouse wheel, you're sent to a different part of the battlefield, giving orders to your support troops or a secondary base.
In fact, this method of navigating the map becomes second nature incredibly quickly, that the more traditional methods of moving the camera about the place (scrolling when the cursor reaches the edge of the map, arrow keys, bookmark hotkeys) are quickly forgotten.
Yet, despite this, most RTS games still conform to the old methods, a mini-map in the corner, a camera that's relatively fixed in position, the only zoom available being one that lowers the camera slightly and alters the angle.
Supreme commander, in an act of brazen showing off, allows you to turn on a mini-map (which is disabled by default) which is actually a second view port identical to the main one you play the game through, so if you want you can even zoom in to ground level with the mini-map. Of course, this kind of functionality isn't without performance costs, which is why it's disabled by default. A mini-map is also relatively superfluous when you're used to zooming out to see what's going on at a global scale.
I sincerely hope that this becomes a standard "can not do without" feature for RTS games, as I find trying to do without in a game of Company of Heroes (which is a very good game) exacerbating at times - so cut off, do I feel, from the big picture, without my zoom.