So far, the JOP40D build's most major fix seems to address the December "event" bug. As some readers found, the previous 4.2 update prevented users from selecting any December dates for birthdays or anniversaries in the People application.
It's unclear whether the Nexus 7 or Galaxy Nexus are in line to receive the update; however, I suspect they won't be far behind. Looking ahead, it seems logical that other devices will skip the first 4.2 build and jump right to this new update.
For what it's worth, the hidden 4G LTE feature still seems to be present in the 4.2.1 update, giving users limited access to the faster data connection on band 4. Whether this was an intentional move on Google's part or an oversight will likely be answered in the next release. It's possible that the recent uncovering of the 4G LTE hack just happened too close to this new update.
As is the case with most updates of this kind, it could take a few days or more before your device sees its respective notification.