This would drastically change our current copyright regime.
As it stands, everything you write has automatic copyright, and doesn't require even the smallest, brokest artist to do anything—or pay anything—in order to ensure that their creations cannot profit others without their consent.
Requiring every work to be registered in order to be eligible for copyright would essentially change our system from a default of "assume anything you find is under copyright, get permission from the creator to use it" to "assume anything you find is free for the taking, and if the creator did register it they'll have to come after you."
In other words, it would mean that anything belonging to big corporate media like Disney and Hasbro would be near-automatically protected, while small, independent artists would be massively more screwed than they are today, with their creations basically being assumed to be available for anyone capable of monetizing it against its creator's will, medium, large, or megacorporation, to profit off of to their shriveled hearts' content.