In an article today, the St. Louis Post-Dispatch's Kevin McDermott provides some helpful context in comparing Illinois' income tax structure to that of other states.  This passage is paticularly important to understand:

Proponents who say Illinois has one of the lower tax rates nationally, and those who say it has one of the highest . . . are both right. Most states have graduate tax rates based on income, like the federal tax structure. Illinois’ 3 percent flat-rate tax is significantly higher than the lowest brackets of many of those rate. However, it’s significantly lower than any of the upper brackets. (Missouri’s range of rates is fairly typical, going from 1.5 to 6 percent.)

Indeed, of all 50 states, we put the fourth-highest tax burden on our lowest 20 percent of earners. It's ludicrous.