Once again, because it seems to be missed by many people, the purpose of a wealth tax as described by Piketty is not to tax the middle classes. It is to tax the very wealthy on assets they have in the jurisdiction. In general, that should be perfectly possible as you can't move land and buildings, for example, nor shares traded in a local stock exchange.
The very rich do not need this money. However the exchequer does, in order to maintain services at their current level.
But in my view, it should go further than this - it should facilitate wealth redistribution through a tax credit scheme, so that people from ordinary families are as likely to succeed as people from very rich ones. This would mean that our society would be run by the best talent, not just those who went to the right schools.
It's painful to point out that this isn't socialism in any sense. It isn't saying that the government should control the means of production, supply and exchange.
When the very rich complain about this, they are basically making the same arguments that the very rich made in Victorian times, ie, they should get to run everything because they know best because they ... err... are very rich.