Very interesting.

I have been doing some research towards automatic tagging lately, and I found several Python project coming close to this goal : https://pypi.python.org/pypi/topia.termextract/ , https://github.com/aneesha/RAKE , https://github.com/ednapiranha/auto-tagify

but none of them is satisfying, whereas Algorithmic Tagging of HN looks pretty good.

I have been trying to implement a similar feature for http://reSRC.io, to automagically tag articles for easy retrieval through the tag search engine.