So then a tax system based on position and not effort would be better? For example if someone makes $100 an hour he pays 40% of taxes, no matter how many hours he worked, and someone who earns a mere $10 an hour he should pay 20%? Personally I actually like the idea, but then you run into other problems... since not everyone is paid per hour. Besides that someone who makes $100 an hour doesn't necessarily have to be more productive than the person at the workfloor making $10 an hour. He might be sitting in his office with his feet on the desk reading the paper.
Not to mention, someone working overtime isn't always more productive than someone who works 8 hours a day. I mean, they may actually work less hard so they can be paid for another 2 extra hours to finish the job. So actually it might be good that people will not be paid for overtime as much as they'd like to be with the current tax system. There is no motivation to spend more time on the work that should be done.
Personally I'm for a reward system in a way of productivity in some way, but then you might have high costs trying to measure the exact productivity (if it is measurable at all) which might make the extra gain in productivity from people willing to work harder not worth it. It's all good to charge taxes in a fair way, but the tax system shouldn't become too complex either.