It would affect their property strategy. At the moment because a hall is built by unpaid volunteers the tax laws in the US allows them to buy and sell properties and not pay a scrap of tax. I think it is similar in Australia and UK.
All of their running costs would still be an expense, but I believe a lot of their expenditure is capital - and we all know they donate nothing to help poor countries - so they would lose some money for sure.
Then all of a sudden we will be meeting back in homes, no need for Kingdom Halls as we would be under attack.