On your possessions above 25.000 (ex first home) on January 1st, you pay as if you will be making 4% the coming year. The tax is 30% over this virtual 4% gain. So 1,2% tax over the amount above 25.000
But mind if you keep the possessions, next year you again have to pay 1,2% over their value.
In the case of profit tax, you only pay tax once over the real profit. (And can deduct loses)
So in case you have 100.000 at the start of the year, you pay
900 tax. (1,2% of 75.000)
Now if you make 100% profit and start the next year with 200.000, you'll pay 2.100 tax.
Let's say you stop here and keep this amount for 4 years on a 0% interest account, you have to pay 2.100 every year.
So a total of 9.300 tax
If there would be profit tax of 30% you would pay 30.000 once. (30% on the 100.000 profit)
So if you're an active succesful investor, the Dutch system mostly is way better.
But now people are complaining it is not fair, because a savings account doesn't return 4%.
Being an investor myself I fear we will get profit tax in the future.