I was prepared to defend this, on the grounds that it's a minor move to improve the allocative efficiency of the housing market*.
The idea here is simple. £1m houses are scarce resources. Like all scarce resources, they should be owned by those that most value them. If someone can't afford the £2500 annual cost of living in a £1.5m house, they should sell up to someone who can.
This idea, though, is undermined by this:
Older homeowners would also be allowed to defer payment of the tax until their death, when it would be paid from their estate. This would in effect turn it into a new inhertance tax.
The upshot is that the tax is neither one thing nor t'other.
These loopholes mean the tax is a weak way of ensuring the scarce housing stock is used most efficiently. It's not a tax on wealth because it leaves non-housing assets untouched. This fact, plus its low rate, means it's a poor substitute for a proper inheritance tax.And on the LibDems own figures, it'll raise only £1.1bn, which is a tiny fraction of the forecast error in public borrowing, so it's barely signifcant in terms of fiscal policy.
It is, then, mostly a gesture. But that's what politics is about.
* I say "minor" because there are so few £1m houses. According to Land Registry (pdf), only 0.57% of the houses that sold in May were priced over £1m, and over half these were in London.The idea that the tax will touch any more than a teeny minority is a glaring example of the "middle Englanderror" - or more precisely, the middle England bare-faced lie.