I'm in the "opt in" group. I mean, organ donation is a no-brainer - you won't need your healthy organs when you're dead - but if some folks have philosophical objections, squeamishness, or selfishness issues, then it's their right to say no, and they shouldn't have to 'manually' change their status.
Admittedly, I lean this way I really disliked the trend of early 2000s credit card companies to sign me up for shit unless I noticed the clause buried deep in junk mail that said I had to call up a fine-print 1-800 number to avoid $9.99 a month in some rewards/insurance/points plan. So, even when it's sensible to make everyone signed up by default, I don't like "must manually opt-out" plans.
But which people matter? Sposes? kids? parents? In which order of preference do the doc's give weight to?
That's usually established (in the US) by state law. "Next of kin" is usually the operative term. Spouse, parents, siblings - they determine when to pull the plug, perform an autopsy, cremate, or confirm organ donation. The decisions usually go in order of spouse, parents, and siblings, then other relatives/friends/government. The Terri Schiavo case (which is flame and ban fodder) was all about the prioritization of spouse vs. parents.
In certain circumstances, even parents and friends can be excluded from decisionmaking without a will. I had a friend pass away a few years ago and his family was unable to access or claim many of his legal aspects (e.g., bank accounts, home, real estate, property) because he hadn't left a will. They had a say in his organ donation and police matters, but not the aforementioned issues.