Why WTFPL is bad for you
The motive behind using WTFPL is obvious: It's novel, it's a bit whimsical, and it dispenses with the stuffiness of normal licenses — but besides the fact that it's becoming an old joke, WTFPL really isn't much better than no license at all, and here's why.
Google Code provides a good explanation[1] of why releasing your code into the public domain is bad for your project. First, people in certain countries (e.g. France) won't be able to fork your project. But more importantly: "it's somewhat risky to release code without explicitly disclaiming liability or warranty; you're potentially opening yourself to lawsuits from accidental damage that may result from somebody using your software."
This is the key, and it's why most software license include an all-caps disclaimer of warranty and liability. The WTFPL doesn't have this, which is why you should choose an equally permissive (but more protective, for you) license, e.g. MIT.
Here's a good comparison of different licenses: http://web.archive.org/web/20090317083515/http://developer.kde.org/documentation/licensing/licenses_summary.html
[1] https://code.google.com/p/support/wiki/FAQ#Can_I_host_code_that_I_have_placed_in_the_public_domain?