It's hard to predict what would happen if negative gearing was abolished. Rents tend to track earnings much more closely than house prices as demand for rental properties is more elastic. For example, a tenant could move back in with his or her parents, choose a smaller property, or even share with friends, all of which would reduce demand.
Similarly, property prices could fall. But bear in mind that I've seen similar, single figure yields in the UK where there isn't any kind of negative gearing.
Oracle, if a large number of investors decided to sell then I'd expect property prices to fall, and rents would probably rise.
The question is then whether that's of benefit to renters. Using the UK as an example again (sorry), the average first time buyer is now 37 and this age has been rising by about one year every year for a while now. What's happened is that property investors have been out-competing would-be buyers at the bottom end of the market, and effectively replaced them. So there's a large population who've been priced out, and substantial falls would allow these people to buy. That might not be great for PIs, but it's good for society.
I agree with .toe that a growth in NG could draw the government's attention. I'm not sure what their budget is, but I'm estimating it to be in the $300 to $400 billion range.
$6 billion of deductions isn't a large proportion of that, but if it rose to $20 or $30 billion then it starts to have an impact.
I disagree about where the risks are though, and I think that there are two that haven't been brought up.
The first is that negative gearing encourages a high leverage. The typical Somersoft strategy involves an 80% loan to value ratio. If one of the Doom and Gloom scenarios played out then a lot of PIs would be in a very painful position.
Removing the incentive to retain high levels of borrowing would reduce levels of risk in the economy it would make more sense to pay down loans.
The second is that I believe it's a form of Moral Hazard. It reduces the downside of a bad investment.