Gamasutra - Features - "Game Taxonomies: A High Level Framework for Game Analysis and Design" Printer Friendly

I'm reading this and so should you... at least if you're in my game design workshop.

Also excellent from this author:

http://zerogame.tii.se/pdfs/CGDClindley.pdf

http://zerogame.tii.se/pdfs/NILE.pdf

http://zerogame.tii.se/pdfs/CausalNormalisation.pdf

Well worth the download.