I'd guess en passant is more surprising to neophytes than castling
Oddly, though Wikipedia deletes programming languages like Factor due to non-notability[1], editors get to go nuts on everything chess or Star Trek without recourse. If you want to know more about why things in chess are the way they are, Wikipedia is your pal.
[1]: http://en.wikipedia.org/wiki/Talk:Factor_(programming_langua...
If you want to start an argument, stop someone from castling through check.