3D user interfaces are one of these - every so often someone shows off something that looks incredibly cool, but is clearly about 10% as usable as, say, the standard Windows/Mac/Gnome interface. They promise to take their new wonderful interface and make it more useful, and that's the last anyone ever hears from them.
And the latest thing to fit into this category of "sounds shiny, can't see the point" is Wolfram Alpha.
Can anyone explain what I might actually want to use it for? Because I've played with it a couple of times and I can't figure it out.