Karajan knew how to kick an orchestra's ass and make it play its most expressively and perfectly, especially with the standard Beethoven repertoire. Next to the Vienna philharmonic, he possessed probably the finest instrument at the time to work with and pushed the Berliners to heights well beyond their so called peers. The mannerisms of his symphonic gesture were admittedly not well suited to certain repertoire (see Stravinsky' critique of Karajan's Rite of Spring), but this is the case with all conductors.
However, the man himself was deplorable in many ways. Obsessed with money, fast sports cars, and fame. He was also very egotistical, which turned many good musicians away from working with him, sadly. As Essa-Peka Salonen pointed out in an interview, Karajan was probably the first name in history that appeared larger than that of a composer on the cover of a recording. Pop star indeed. He also married a jew, lulz.