A nightmare had followed me for decades on both PCs and Mac. I am working on a file and decide that it should be called differently or be placed in a different folder. So I switch to Finder/Explorer, look for the file and try to rename or move it only to get the Operating System shouting at me: “You can’t change the name while the file is open!” #@%$! So I go back to the program, close the file, go back to Finder, rename or move it, and open it again. Whew! Well, in Windows, (and in Mac before Lion), you could also Save As …, but it created an unnecessary duplicate: with your file now being in two places or with two names leaving you forever wondering which one is right? Finally, this nonsense is over with Mountain Lion.
In most Mac apps (though not yet in Microsoft Word) you can now change the name or move the file without closing it. To the right is a screenshot from Byword‘s top bar.
Katie and David discussed a related Save As …/Duplicate … feature (re-)introduced in Mountain Lion in their “Working with a new cat” episode (MPU #101), but they did not talk about this in my view more revolutionary feature.