QuickMix is now called Shuffle and works in the same way. Shuffling your stations provides a more diverse mix of music by randomly playing songs from the stations in your list.
Click on Shuffle at the top of your station list to enter this mode. You can click on the arrow under Shuffle to select Shuffle All (which includes all of your stations), My Shuffle (which you can customize), or select stations from a particular Genre.
If you select My Shuffle, then you can choose which stations to include by checking the box next to that station name.
To turn off your Shuffle, simply select a station to listen to from your station list.