Ok, so maybe that title was a bit of a hyperbole, but this is one of my biggest pet peeves on the Web right now. Joshua Schachter has a comprehensive analysis of the concept of Short URLs and the myriad problems associated with it, but the biggest one for me is the fact that the true URL is hidden from the user. As a user, if you want me to go somewhere on the Web, you better damn well tell me where that is first, instead of expecting me to blindly trust you. Another primary reason short URLs should (almost) never be used is their potential as a massive attack vector for hackers.
In order to get around the “opaqueness” problem, there are several browser extensions that will automatically expand the short URL — my favorite is Explode — which goes a long way toward mitigating the negative impact on user experience, but doesn’t solve the fundamental design flaws or security concerns.