Actually I've heard many people say (and I agree at least partially) that the "Spiegel" is quite "left" in the political spectrum. One example would be "Im Zweifel links" by Augstein and Fleischhauer.
IMO the "Der Spiegel" changed a lot since the death of Rudolf Augstein. Consider for example that for a while their assistant editor in chief was the former assistant editor in chief of "Bild". Which is very popular and very much on the "right" side of the political spectrum.