I think versatility and the absolutely easy on-ramp are Python's killer app. The 'absolute noob' can start writing simple python code after 5min of introduction (you can start using it like BASIC), while the 'senior dev' can still write python after years and isn't turned off or overly constrained by the language. (Yes, of course, warts exists, no language is perfect)

And regarding Django being the killer app: There are other people for which numpy/scipy is Python's killer app, for example.