Theoretically, wealth taxes are one of the most progressive taxes out there which also give the best economic incentives for growth. Income taxes discourage earning money, sales taxes discourage consumption, capital gains taxes distort / discourage investment. Wealth taxes encourage people to make the best return from their assets, and if they can't do it, sell it to someone who can.
It doesn't work for three reasons:
a) The *truly* wealthy get hurt the most by far. The ruling class will not let anything like this to happen. Other posters moaned about this hurting the middle class is a load of baloney. A small wealth tax would allow for a significant reduction in income taxes, sales taxes, or deficits.
b) Unless all jurisdictions do it, liquid capital will just move elsewhere (which is probably why wealth taxes are only widely used for real estate).
c) Some assets are hard to value. There are ways of doing this, but they are all ugly.