What I perhaps meant to say was that the WTFPL isn't a sensible license.

The problem is, for anyone who wants their software to be used by anyone, the WTFPL is dangerous since it doesn't include a disclaimer clause. You say "one of those "I'll sue you" countries", but the reality is that basically every country is potentially an "I'll sue you" country, and if I ran some open source project, I wouldn't want to live in fear of being taken to court because I failed to choose a good license.

I am not telling you to stop using WTFPL because I don't like it or something (although I don't). I am advising you to stop using it because it puts you at unnecessary risk.

If you like the ISC license, just take it and delete the "provided that the above copyright notice and this permission notice appear in all copies." bit. The license is widely accepted, provides adequate protections to you, and places no restrictions on the users (if you delete that one clause).

I don't see why anyone would want to use the WTFPL. The OSI rejected it because it didn't add anything new to already existing licenses (it's just a more ambiguous and less safe version of some permissive licenses). It's plain to see that it was written as nothing more than anti-GPL satire and should be treated as such (i.e. not seriously).

Sure you can use it, no-one will stop you. But I would never want my name on a project without a real disclaimer and neither does any software author, really.