This proposal would serve as a form of presumed consent, in that you would be presumed by the government to have consented to "donate" your organs unless you specifically declined, or opted out. This comes on the heels of a controversial New York City "organ harvesting" pilot program, and other previous, proposed New York legislation.
The practical implications -- and dangers to your life -- are staggering. You could die so someone can steal your organs.
First, such a system would shift the burden to you to prove -- after you are dead -- that your wishes were against any donation.
Secondly, such a system assumes that your documentation for "opting out" would be in order, accessible and respected. Ultimately, your last wishes being respected will depend on the willingness of others to obey the law and your wishes. This presents an insufficient deterrent to violations of either the law or your or your family's last wishes.
Finally, any system making organ donation "automatic" raises the risk that people will be allowed to die in order to satisfy someone's zeal for a particular organ. At the very least, there is the risk -- which you bear and which puts you in mortal danger -- that a hidden agenda, valuing you as a dead organ provider over a living, breathing person, would be paramount at the moment when life-or-death actions affecting your life would be made.