Regardless of how you (one) feel about the merits of Siegel and Shuster vs. DC owning Superman or Simon and Kirby vs. Marvel owning Captain American, I think this new law is a bad one. It does two things. It "steals" from the public domain, by unilaterally extending the copyright. And it restricts a creator from having full ownership of his property since apparently (s)he cannot sell all rights to the creation, but must retain the right to reclaim the character. (And sets the precedent that other rights could later be reassigned by law rather than contract.)
If I understand the Superman case correctly, the "right" for certain creators to reclaim an old copyright isn't something that current creators will have to deal with. This right is only an option that applies to certain people whose older work falls within a narrow window of time set by one of Congress's many recent revisions to copyright. Basically, the basis for allowing creators of these older works to reclaim their copyrights is based on the fact that Congress has (like Darth Vader) "altered the bargain" when it came to the creators' old work.
Back in the Golden Age of comics, the creators would sell their work to the publisher for X number of dollars, in exchange for the publisher controlling the copyrights for X number of years, after which it would fall into public domain. Later, Congress extended the copyright lengths, granting the companies longer and longer control of the copyrights. Since the companies were now essentially getting extended use of a property that they hadn't paid for as part of their original contract with the creators, Congress decided to balance things out by allowing those original creators to "reclaim" their share of the newly extended copyrights if they filed within a certain timeframe.
It's a complicated mess, but at least it only applies to old work whose copyrights have been artificially extended. It's not going to apply to modern-day writers whose new work is being created and sold under modern copyright rules... Unless, of course, Congress decides to step in and rewrite things
again, as they seem compelled to do anytime Mickey Mouse gets close to entering the public domain.