I agree Cogito and Mamma, it is unfair for exactly the reasons you say - it is also, ultimately, a tax on living in London because that is where 90% of properties over £2m will be - and I realise that value is massively out of kilter with the rest of the country, as are lots of people's incomes in London, but £2m in London really does not get you an amazing house and mean you are cash rich. In some areas it would get you a 3 bed terrace, even less in others. Many owners of these will be stretching themselves to the limit to pay for them, in the same way as everyone else in the country is, it is all relative.
Why should someone living in London in a £2.1m house (with, say a £1.5m mortgage) pay a mansion tax when someone living in, perhaps, Yorkshire in a grand £1.9m mortgage free (proper) mansion with a yacht, horses, art, jewels, not pay? Why should you pay tax on something you have bought out of already taxed income (twice taxed if you count stamp duty)? Many people living in properties they have owned for a long time may find themselves caught by this just as a result of the property market rising and simply won't have the income to cover such a huge rise in costs, they would be forced to sell and downsize their homes or relocate. And I can hear people saying - well sell then, but how would you feel if that was your elderly parents? £2k+ more tax a month is a lot for all but the proper super rich to find.
As an aside, I wonder how this will be implemented...how will they value houses unless they have actually been bought/sold at £2m plus? I can imagine lots of legal challenges if valuations around the £2m mark are made without any concrete evidence to prove a value. Also, what will the effect on the property market be? It would inevitably push many people out to the areas just outside London, increasing values there - good or bad thing? I'm not sure.
I agree the wealthy need to pay more tax somehow but this really isn't a good way to do it. It is such a cheap shot for Labour, going for the popular "hurt the rich" strategy that many outside of London may support but won't make any difference where the Tories have safe seats, ie where the tax will hit hardest - so they won't lose any votes with it.
The Government need to find a way to get more from the Arabs and Russians at the very top end of the London property market, the ones buying houses that are £10-£50m and paying nothing, not living there in most cases...increase their stamp duty and close the loopholes perhaps? Would be interesting to know how much that could raise.