The greatest hidden feature of Python (for me) was its standard library. Its sheer size and completeness, I should add.
Yes, I have heard of "batteries included" and all, but after working with Python for a year I still felt that the size of its library has been "hidden" from general programming public.
You can add the number of Python implementation to your list of "hidden" features as well: IronPython, CPython, Jython, PyPy, unladen-swallow, Python3 - and all of them are of pretty high quality!
Yes, I have heard of "batteries included" and all, but after working with Python for a year I still felt that the size of its library has been "hidden" from general programming public.
I love Python.