Unfortunately some people, especially those at the beginning of the thread, don't see anything other than 2 million and think rich, loaded = no empathy.
^ This.
As others have already said, firstly you are in effect taxing people again for something they've bought with income they've already paid tax on.
Secondly you're moving the goalposts for people who may be living in high value properties because they were lucky enough to buy at the right time or inherited properties.
Lastly once you introduce a tax like this, the risk are the cutoff limits slide just like the 40% tax bracket has. Once upon a time I'm sure 40% seemed like a tax only the wealthy paid except now it's paid by a lot of middle classes which was never the original aim.
The same principle applies here. They set the limits at £2m now....but what in 5 years time when Labour need another spending spree that sounds good on paper? Ooooh we'll tax mansions over £1m and pay for 500 more nurses....i mean nurses have to be better than the rich living in their million pound houses right? Another 5 years....£750k houses! Those rich bastards. We can now pay for 500 teachers! And so on.
Frankly we need to get better at what we spend already rather than continually think that throwing more money at a problem will solve it.
Labour chucked £64 BILLION on a computer IT project for the NHS. What have we got for that? We can now make appointments online at some hospitals..... Whoopee. That was worth it then!