I sometimes backup my songs bought from Itunes to an external harddrive, and it used to be very easy with the "dont replace"-button, then it only copied all my new songs. I didnt have to try rember what I have bought the last few weeks and then copy only those, or replace the entire library ( several 1000 songs after many years with Itunes usage ). The last option is quite time-consuming. So I really like to have the "dont replace"-button back.
For those of You that want a work-around , there is a programs that is like the old windows/Total commander
( for those who has used windows ) : Xfolders 1.6 ( free ) , that is 10 times better than Finder . By example 1 function that it has :"compare folders" under the edit , will compare the chosen folders in "both windows" ( it is divided into 2 do sides wher You can choose 2 diiferen volumes or folders in the same volume , so You can copy between them ) and mark the onse that are different in size.
NB It says on the page that it has not been tested and doessnt support Snow Leopard , and use on own risk. I have not had any problems with it
picture : http://www.kai-heitkamp.de/xfolders/xfolders_en.png
review : http://alternativeto.net/software/xfolders/about