First off, the thing I love about it is that it should(although it doesn't) make the liberals happy because the rich people who make more money are still paying and contributing more money in taxes then those who don't. I also love that since it is all proportional, it preserves and encourages others to be a successful and not settle for an entry-level job. Those who go to college and walk the extra mile should be paid more than those who didn't. By having the percentage across the board, everybody gets that same amount of money taken out of them for every dollar they make.
The socialist ideal is to make the poorer pay no taxes and have the rich pay a high portion of what they make, this effectively cuts down the hourly wage made by those who make more money and increases the hourly wage for those with entry-level jobs which demonizes success. A person working at McDonalds which requires no education and hardly any experience, shouldn't be paid the same amount nor should they be paid more than let's say, an engineer who had to spend thousands of dollars to go to college so he could have that job.
What's also great is that since everybody is under the same tax bracket, any President who tries to mess with that tax system will not affect only some people but ALL people. This will make sure that the people keep the government in check. If we were to divide up Americans into different tax brackets, that gives government the authority to pick on a minority of Americans(the rich) and use the poor people votes against them. I find this turns Americans against each other. With everybody in the same tax bracket, if government decides it wants more money, it has to take from everybody's pocket which nobody would want. This is better than bribing the poor to vote to take more money away from the rich and use that money to give to the poor so they can bribe them again into doing whatever they want to.
All in all, it preserves motivation to be successful, and rich still pay more money. And also, in no way shape or form is it fair to say that because the tax system doesn't hurt the rich then it's not a fair tax system. Why are we supposed to hurt anybody?