Andrew Fieldhouse of EPI says less progressive taxes alone don't explain the huge rise in inequality in recent years. But what low tax rates spur rich people to do might explain more of it.
Compared to peer countries, the U.S. taxes the rich quite heavily. But it doesn't spread what money it collects as equitably.
Once you take state and local taxes into account, the U.S. tax system doesn't hit the top one percent all that much more than the middle-class.
It's usually conservatives like Mitt Romney you hear bemoaning the 47 percent who don't pay income taxes. But liberals who admire Europe should bemoan that number too.