One of the things I like about Slic3r is that it adds the entire print settings to the G-Code file, so that it is possible to reproduce them later on, I've found this to be extremely useful.

I was thinking that it would be nice to be able to, from Slic3r, select an existing G-Code file to change the print parameters according to the data stored on that file, easier than going item by item and less risk of making mistakes or omitting something.