I hate the process.
I understand the "do your own math just in case you don't trust the government" but it still sucks. I hate the loophole-ridden laws that complicate filing. I hate that it's bad enough it can support a giant tax-preparation industry. I hate the added computation that the federal government's unwillingness to deal with domestic partnerships introduces. I hate how the process not only makes it easy to cheat on your taxes, it practically encourages it. I hate the huge amount of unpaid tax that's sitting out there.