Nothing except for the limited size. I believe even today, nothing exists today for temp storage that have the convenience of diskettes.
USB Flash Drives comes close, but cheaper versions can in rare cases have firmware "virises". On a diskette, just do a format and all issues gone. Also I never even thought twice about mailing a file on a diskette expecting to never see that diskette again. Flash Drives, I still would like to get it back after mailing it out :)
No, to write to a CD, you need to use specialized software, not just a simple copy.
Plus what about copying to replaced a files already on the CD ? I believe you need to clear it first then rewrite everything back. CD are about as inconvenient as a media can get.