NTB, I think my main problem with it, is that it has not been implemented in a western society yet, it's basically still a trial system. The benefits are all based on theory - sure it's been implemented in Latvia, Estonia and Russia, but, Estonia, Latvia and Russia wtf?
The current system works, although it probably has alot of room for improvement. The notion of paying a same proportion sounds good, but when you're earning less and are being deducted the same percentage rate as a higher income earner, you're paying a shatload.
30% when you're on 20,000-30,000, as opposed to 30% when you're on 100,000+.