The complexity for income tax law stems from tax breaks. There are all these different ways for people to pay less in taxes, but each rule for paying less has its own set of instructions and there are criminal and monetary liabilities for filing incorrectly. So it can be said that if you just pay your income taxes as they come and don't claim any deductions, you are essentially over paying in taxes. Another problem problem that arises from this system of tax breaks is that it allows people receive more or less in deductions based on their life situation which is inherently inequitable.

Tax law doesn't have to be so complex though. For example, you could just have a simple income-bracketed flat tax that is just at a lower rate than current taxes without any deductions possibly. Or possibly better yet, you could get rid of the income tax all together and just use a federal tax on sales, real estate and money transfers out of the country. Granted, this would give the government more direct control over how much money is being collected on a daily basis and could result in some people feeling like they need to have a party with tea, but assuming we do live in a democratic society, something like a sales tax would be more understandable and visible to everyone (as the rate would be printed on all of their receipts) and it would probably lead to more oversight of the tax by the taxpayers rather than everyone just being concerned with paying less than they are asked to each year without getting audited.