One can argue that it should be the opposite of an income tax.<p>If the value of your house goes up because you remodel the kitchen, then it seems fair that you should be able to pocket that value. You did the work, so your taxes should not increase.<p>If the value of your house goes up because the government opens a new light rail station nearby, then you did nothing to earn this, so it seems fair that society should be able to get higher taxes from you.<p>In a speculative bubble, you did nothing to earn the extra value, but---arguably!---neither did the government, so it is less clear cut.