I was greatly pleased to see that xkcd plugged organ donation. I was even more pleased to see it done using Legos. Personally, I'm a big fan of organ donation and hope to see as many of my own functional tidbits re-purposed when I am done using them - even if they are in such crummy condition (not an unlikely scenario) that they are only useful to an introductory anatomy class. This metaphor does a great job of addressing the general "ookiness" people feel about taking one person's organs and slapping them into another. Like all metaphors, it falls apart when you try to extend it beyond the argument for which it was designed.
The Lego metaphor does not address the selfish*** religious objection or the conspiracy theorist. The religious objection is based on resurrection/afterlife stories in which the dead body is either reanimated or used as the model for a new body in a new life. Essentially, the deity of your choosing is going to rebuild the Lego house, except that, if you donate an organ, some of the pieces will be missing****. Depending on the piece, that could really ruin the afterlife.
Conspiracy theorists worry that doctors are less likely to try to save people who have signed up as organ donors. While the ethics and practicality of the approach to medicine this would require are laughable, the Lego metaphor is not far-fetched. I have decided to scrap one Lego creation in order to get pieces for another.
In short, the Lego metaphor works where it works: ookiness, not religious objectors or conspiracy nuts.
*If it did not have a bar we would first develop the definitive baseball euphemism lexicon and then fight to the death, just for something to do, which I would unfortunately** win.
**Because I cannot draw.
***Selfish in that they are denying a benefit to another person for their own perceived gain.
****Like after I repair my lawn mower.
