Maybe the core WordPress developers should add an entire nag page, which is displayed when an administrator logs in and there WordPress version is out of date. The page could have a huge update button followed by a list of vulnerabilities in the current version their running.

WordPress should also assure people that their site isn't going to break when they upgrade. We (developers) could create scripts that check plugins and themes for functions that would cause problems. We could introduce site cloners, so people could test out their actual site before updating the real thing. We could state some facts from the plugin repository, such as all your plugins work in the current version of WordPress. I think the WordPress backend could do a better job of forcing people to do the smart thing and upgrade, as well as informing them about problems that will or will not happen when they upgrade. I think even a "more info" link in the update nag would help people over come their fear of updating. I don't see why administrators aren't getting more information about the functionality and compatibility of the software and extensions they are using in the backend. Take for instance Linux, I think there is a reason it tells be about compatibility issues before I install or update... it creates informed decisions, that create fewer problems.