The problem I see is that people vote (or hate) personalities. The truth is that the election process has little to do with policies. Who is expressing them and how those ideas are marketed matters even more than the content itself. In this context, a negative vote would probably be directed to someone (and not something) who people do not like. You would probably get pretty horrible policies just because the guy promoting them seems to be a good chap.

I believe however that the rationale behind negative voting is sensible. But, on the other hand, it'd be just a minor patch for a perverse system.