I am tempted to counter that the first two attributes, unique and rival, would exclude the third, re-giftable, and vice versa. My thinking is that anything that appears to fit all three is actually functioning like a gift certificate (or money). For example, DNA analysis is unique and rival but once it is run for a specific person it is no longer re-giftable. Likewise, before it is run, while it is still re-giftable, it is not actually unique…it is only potentially unique.

The condition “economically hard to produce” would also seem to fit the above analysis. I interpret this condition as “requiring some scarce resource”. A scarce resource once used for one product or service can't then be reused for a different version of that product (customizing to someone else). So again it would seem that you must choose between things that are certificate based, providing access to some resource, and things that have already drawn on that resource and are now specific to a given person.

I will be curious to see what other people come up with…