The notion of deductions (Schedule A) is complicated enough. I did not know, for example, that state income tax was deductible (pre-tax reforms). If I hadn't just happened to use a tax software (I almost never use one), I would have been overpaying in taxes.

Now of course, once I learned this, Schedule A was a breeze. But it's not something I expect a common person to know, and it's unfair that some of them may be overpaying in taxes due to a (nontrivial) piece of knowledge.

And then I'm always wondering if there are any other deductions I should be aware of.