Wealthy people's stock in retirement accounts would also not be taxed. This can be considerable: Peter Thiel's Facebook investment was made in an IRA.

I imagine there'd be some net worth number, excluding retirement accounts, that policy wonks could work up. You draw the line between "wealthy" and "regular" there. Or, more likely, several lines because there would be wealth brackets similar to income brackets. Without that it would be a regressive tax.

Why not just tax when someone SELLS the stock, or leverages it for a loan instead? You know, when they actually use it?

I'm actually against property taxes, or any kind of tax where you risk losing property just because you managed to live another year.