I think the biggest problem is definitions. Keep in mind that copyright means very different things internationally. Let's compare the CC0 text to this one. Note that I'm not a lawyer, and this is mostly conjecture, but pulled from a reasonable amount of reading I've done on the subject.
Part 1: This talks about what copyright means. Given there is no universal definition, this clarifies exactly what is being given away.
Part 2: This is worth quoting.
. To the greatest extent permitted by, but not in contravention of, applicable law, Affirmer hereby overtly, fully, permanently, irrevocably and unconditionally waives, abandons, and surrenders all of Affirmer's Copyright and Related Rights and associated claims and causes of action, whether now known or unknown (including existing as well as future claims and causes of action), in the Work (i) in all territories worldwide, (ii) for the maximum duration provided by applicable law or treaty (including future time extensions), (iii) in any current or future medium and for any number of copies, and (iv) for any purpose whatsoever, including without limitation commercial, advertising or promotional purposes (the "Waiver").
Notice how this is unambiguous, and takes into account many different cases. Compare that the the blog post:
There is no need to email me for permission — use my content however you want! Email it, share it, reprint it with or without credit. Change it around, put in a bunch of swear words and attribute them to me. It’s OK.
First of all, what does use mean? Does that mean I can share it with others, and they have the same right to it? And is he allowed to take it back?
What happens if he decides he does not want to have people giving away his books anymore? He's not said that he can't change it in the future. CC0 explicitly says it last for as long as legally permissible.
Furthermore, what about in countries like Germany, where you can't actually give up your copyright? CC0 has a fallback (part 3) that essentially says "if I can't give up my rights to this, this is the license I am releasing it under", then provides a license with no restrictions.
I've been packaging software for Debian recently, and dealing with copyright stuff. When you start distributing data internationally for a large organization, you really want to make sure all of your ducks are in a row.
