That’s why I examine the disks I’m borrowing from the library before I leave with them.
Please tell me you don’t do that while still standing in line. And please please tell me you don’t lecture the librarians if you find a scratched one.
No, if there’s a line I step to the side so as not to interfere with traffic flow. Minor scratches I ignore, but if something major is visible I might point it out to them. I don’t know why I’d “lecture” the librarians, I’m pretty sure they didn’t do the damage. I might inquire if there are any notes on file, or if there is an undamaged copy.
Maybe steel wool works better.
Reminds me of this. Actually, this is a site showing how to defraggle your computer’s hard disk, but this link goes to the step where he polishes the disk surface with steel wool. Looks like it ought to work just as well on CD/DVD too, ya think? 
(ETA: If you’re interested, the actual defraggle happens on pages 4-5 of that site.)