Reading the comments to the answer of this question, it seems that the Unlicense presents legal problems in countries like Germany. This is because German law doesn't permit simply waiving all copyright and placing your work into the public domain.
The WTFPL avoids this by going a similar route to more traditional free licenses like MIT, namely explicitly granting all possible rights without actually waiving ownership of copyright. Now, this means it has some problems of its own, for instance the lack of a liability disclaimer. Ignoring that, however, is the license as it stands applicable globally? For instance, can it be used in Germany to allow anyone to use the software for any purpose without fear? If not, what jurisdictions would have a problem with it?
To clarify, by "problem" I mean the law preventing the license from applying as stated, or severely violating its spirit. For instance, it appears that Norway has a law that forces anyone performing CC0 music to pay a few to a government agency, which clearly goes against what the license tried to achieve; Germany, from what I gather, simply doesn't support putting works into public domain, so you could sue someone for using a Unlicense-published work, which goes directly against the license as written.
Here is the full text of the license for easy reference:
DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE Version 2, December 2004 Copyright (C) 2004 Sam Hocevar <email@example.com> Everyone is permitted to copy and distribute verbatim or modified copies of this license document, and changing it is allowed as long as the name is changed. DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION 0. You just DO WHAT THE FUCK YOU WANT TO.