Why was Kalecki a great thinker?
Kalecki is often regarded as one of the most influential and widely read Marxist economists of all time.  He produced a large body of work, which spanned many fields including macroeconomics, monetary theory, labor economics, industrial relations, and political economy. He wrote at a time when neoclassical economics dominated academic discourse and challenged this view by developing theories that were consistent with the existing economic data from both theoretical and empirical standpoints. 

Another important reason why he was so influential in academia could be due to his impact on some leftist movements in Poland in particular and Central Europe generally. As a member of Polish socialist party's ruling politburo, Kalecki was forced to resign following publication of "Political and Economic Considerations concerning Communism" in 1946 where he criticized Soviet model and proposed free enterprise based alternative system (with some state ownership) in post-war world