I'm listed as a donor. I've also donated samples to the marrow registry so that I can be considered for those in need of such. But it should always be my choice to do so. I refuse to disrespect or shame somebody who isn't willing to donate, because it's a weighty decision.
I also fear that mandatory organ donation might lessen life saving efforts bestowed upon those in need of urgent and extreme medical care because it might be cheaper or more "beneficial to society" if they go ahead and harvest organs than perform those extreme life saving measures. Unless some clause is included that maintains the right of the NOK to make medical decisions indefinitely in absence of a DNR, I couldn't get behind any sort of mandatory law. Too likely to see abuse, I fear.