Walking hand in hand with the low taxes on the wealthy, 401(k)-type plans would not exist if the tax breaks hadn't encouraged people to use them. This is good if it encourages people to save and invest for the future. However, it can also lead to people blindly shoving money into investments they don't understand or overpaying for a mutual fund manager to do what amounts to the same thing.
As previously mentioned, municipal and state bonds also enjoy the benefit of being on the right side of the tax code. Not unlike the mortgage deduction, the muni tax-free status gives states enough money that they often forget to balance budgets and lean towards overspending and refinancing. This puts states, and investor capital, in danger.