If we were to have some sort of wealth tax as been kicked around my Dr. Piketty, there will be a natural push to avoid the tax. This could take any number of forms, some benign and some less so.
(1) Human Capital. Let's take two smart young high school grads. One is a gifted mechanic who starts his own machine shop. It bristles with new 3D printers and CAD-CAM equipment, as he plows his profits back into expanding his facility over the seven years since high school. He's now worth $400,000, having done a great job making himself useful to a variety of businesses in town and creating a thriving small business.
The second, equally smart grad, has a MIT engineering degree and a dual MBA-JD from Harvard. He's plowed $300,000 into his education, but has no financial assets to speak of outside of $150,000 of student loan debt.
The first guy has property tax to pay on his machine shop. The second guy gets a tax break from his student loan interest and pays no taxes on his human capital, at least until his training, degrees and connections start paying dividends in his paychecks.
A wealth tax will be tougher on capital-intensive industries, but spending money on schooling will be encouraged in such a universe.
(2) R&D for a private firm. If a company plows money into research, it will see its stock price go up if the research is proving promising. A wealth tax will extract a percentage of that market value well before any positive cash flow comes from the project.
However, if the research is being done for a firm that is not publicly traded, it will be hard to value such future projects, especially if the firm isn't trumpeting how its research is going, waiting until a finished product is ready to go public and rake in the cash on their new gizmo or wonder-drug.
The private firm is at the mercy of whatever a government appraiser might think of the research if a wealth tax is to extend past just publicly-traded corporations; given the bad PR of private equity firms and hedge funds, some evaluation regime would wind up getting cooked up to capture that tax base. If the appraiser likes the product (or is disposed towards high estimates in order to give his bosses high wealth figures to tax), they pay taxes on that higher estimated value. If the appraiser (or his boss) likes the firm, he can low ball his valuation and save the investors a nice chunk of change.
(3) Political donations. If Mr Fat Cat gives $5M to various political causes, he won't be taxes on that money that is now longer in his bank account. However, if he has chooses his beneficiaries wisely, he will have roughly that amount of goodwill in politicians that will help him in various ways in the future.
(4) Charitable donations. Giving away part of your fortune to charity can make friends as well as political donations. The goodwill generated from those donations can help as much as a direct political donation. Also, the charity could be one that promotes causes that one's political allies will like, being a political donation in all but name.
That might sound like a spot for conservative fat-cats to tweak the system, but foundations tend to be critters of the left; even if the donors might not be all that liberal, the nature of where charity dollars are spend leans left. The money that the reactionary barons of a century ago donated (Ford, Rockefeller, et al) oft goes to help promote a number of liberal causes.
That's not a complete list of what could be done to turn taxable wealth into intangible wealth that would fall outside the taxman's reach, but at least a start for today.