The Lib Dems are just proving themselves, time and time again, to be a complete joke of a party. They know they aren't going to get elected, so they come up with stupid policies and personal attacks.
I won't waste time talking about personal attacks, which are generally a sign of losing the argument, but the 'mansion tax' proposal is so poorly thought out that it would be completely unworkable. The main problem is based on ability to pay, what brought down the poll tax. People can (generally) afford to pay income tax, since obviously they are getting an income. Corporation tax is a tax on profits, and companies will move around if need be. Sales taxes are based on being able to afford a product. Obviously if any are too high, there may not be an ability to pay, but that's another issue.
A tax based on the value of a house ignores who lives there, and their income. Can a pensioner pay £2,000 a year just for living in a nice house? With their income already low (and reduced potentially further due to Brown's raid on private pension funds), probably not. It just becomes a jealousy tax, saying that success in life is some sort of bad thing. Perhaps it isn't even due to that; they've just been caught up in the housing bubble.
How do you measure the value of a property? How often? Assuming a tax is collected annually, that involves independent valuations of many houses, including some that will be below the £1m threshold. Would the threshold be raised with inflation? Would the valuations be made by the private sector, or would government officials do it? Would the council give the valuer a bonus for saying it is valued at over £1m? Hayek's Road to Serfdom comes to mind once more.
When you sell a house, you have to be optimistic at first, then pull your price down until you find a buyer. That's the nature of the housing market; in no market is information perfect. If I was to sell a house, would I have to get a state valuation, or even possibly an overpriced valuation due to the huge increase in demand? As usual government intervention distorts the market.
I'm sure there are many other ways that this proposal could be ripped apart. But it's simply an unworkable proposal that brings back the pre-Thatcher view that generating wealth is bad, and should make people question whether Vince Cable is that economic guru they thought he was.
None of this actually matters though, because the Lib Dems will never gain power to implement this crazy idea.