Reply #96: Marginal tax rates are pretty easy to understand, actually.

[Gormy Cuss]

I think the reason many people don't understand them is that when they self-prepare taxes, they just take a number from the tax table -- no need to compute the tax on each level of taxable income as separate items. Those who have others do their taxes, same effect -no need to compute the effective rate.

If on the other hand you're self-employed or otherwise need to figure out estimated tax payments each quarter, the tool used to estimate those payments lays out the rate at each income tier and it's easy to understand.
There's a chart here: http://www.irs.gov/formspubs/article/0,,id=164272,00.html