Why go straight to a wealth tax? In the US, there exists a comprehensive system for measuring economic activity and translating it to income. To measure wealth would require a huge invasion of privacy (not only of the wealthy but the 'suspected wealthy') and a whole new system of identifying and tracking things of value.
Why not fix the income tax? At least try fixing it using the existing system, before creating this huge new boondoggle.
Hint: the top income tax bracket is about 400K. It was 500K in 1913 (~13 million today adjusted with conservative, CPI-based inflation figures). There's your problem. Nobody should be making 10 times more than the top bracket in a year. If they are, you add a new bracket and skim an extra few percent. Right now, the wealthiest make ~10,000 times more than the highest income tax bracket, plus they are typically paying the capital gains bracket, which is even worse, so they end up paying less than doctors and engineers.
When politicians clamor for a wealth tax, it's because they are either A. clueless or B. don't actually care about the problem, but want more power and votes. I think it's the former for Bernie and the latter for Warren.