In an annual report to Congress, National Taxpayer Advocate Nina Olson declared the complexity of the tax code to be the most significant issue facing taxpayers:

It obscures comprehension, leaving many taxpayers unaware how their taxes are computed and what rate of tax they pay; it facilitates tax avoidance by enabling sophisticated taxpayers to reduce their tax liabilities and provides criminals with opportunities to commit tax fraud; and it undermines trust in the system by creating an impression that many taxpayers are not compliant, thereby reducing the incentives that honest taxpayers feel to comply.

The infographic below produced by Olson's office shows how burdensome complying with the code is.