Not necessarily, I have had dvds that play fine on one dvd player and the pc but not on my other (more expensive) one.

Agree that poor copying will be the fault of some of the issues. However not all.

Sometimes the DVD player itself may be compatible, but have a poor quality laser and lense that has difficulty reading the data.

There are also countless people who treat DVD's very poorly, and allow muck to get on them, which consequently scratches them, and turns them into nothing more than coasters.
Despite the marketing hype which people fell for re alleged durability of cd's/DVD's, they are actually the least durable recording format out there.

I would think that between opening a DVD case for the first time on christmas morning and placing the dvd in a dvd player seconds later, there's little scope for dropping it in a hole full of razor blades and horse dung.

Fingerprints can be enough to cause a player not to be able to read the data.

The data is recoreded in Binary in a series of bumps and lands. One fingerprint on a christmas morning would not be enough to stop the disk from working completely. Their would need to be some sort of damage to the bumps and/or lands in order for the disk to be faulty. In the case where one disk is working on a DVD player but another disk isnt working on the same DVD player; formatting isnt an issue, it is undoubtedly shoddy copying.