I'm not sure how hard this would be to implement, but a feature that I am looking for (and have yet to find) in a program is multiple disk spanning with direct file copy (without any proprietary backup formats). If the program could somehow rearrange the files to fit onto multiple disks to maximize their usable space (and obviously it doesn't have to be perfect it would be extremely useful. To have the ability to only copy to one disk at the moment seems a bit limiting. I have many large folders with multiple subdirs that exceed a single DVD which I need to back up - to tag everything separately is a huge headache. I would use a proprietary backup program to do so but I'd like quick access to the files without a full restore.
Anyhow I think it'd be a great feature to have some kind of simplistic disk spanning if you could work it in.
I see no way implementing spanning using plain file copy that way it would be practical. You may run it once, but later when you want to do incremental backup, the program needs somehow to keep track what files are on what disks, file sizes etc.