A flat tax seems to be a simple way of solving a lot of problems. The first complaint from many people would be you shouldn't have rich people paying as much as poor people. That can be fixed. First, take away every tax deduction that exists now.
Deductions tend to provide the biggest benefits to people who make the most money.
Apart from saving us from the patronizing television commercials that run at this time of year, the government will be saved all the book-keeping involved. We've become a nation of busybodies. A flat tax would put a lot of accountants out of work, or at least direct them to more productive work than filling in income tax forms. It takes a lot of brains and hard work to become a chartered accountant, or the other kind that always advertises they do just as good a job. What a waste of all that talent, figuring out ways to take beat the tax department.