Execution speed...
when you play with big quantities of data in science, the speed is usually limited by inefficient code, not by inherent properties of the language. When I crunch my 5 GB dataset, making a for loop a little faster won't make my code run in a reasonable time -- but moving to a sparse data representation or avoiding the loop altogether will. Python makes those things easy, that's why it is a game changer for science.
Biting the hand that feeds IT
