The research also goes on to look at how game design has become more of a storytelling vehicle rather than just simply providing the player with increased level of difficulty as they progress.
as a side note, I also have begun to put some emphasis on how in game tutorials have almost killed off the user manual that used to accompany most games (although that is a personal annoyance on my behalf so i'll leave that there xD)
Just a sidenot, as to not derail the original topic, but that is an EXTREMELY good point. I once read, I believe it was Cliff Bleszinski, talking about creating a game where no instruction manual is needed at all (that game was Gears of War, btw).
To me, instruction manuals were always an extension of the product, particularly in the 80's-90's where memory was so limited in gaming systems, that they placed the intro storylines in the manuals themselves. Heck, a lot of story info was exclusively found in there (Mortal Kombat profiles, Zelda profiles, maps, etc). Speaking of Halo, I still keep the original Halo: CE manual, simply because it said Spartans were designed and engineered specifically to capture enemy Coventant ships and weaponry and finding their homeworld before they found ours. That was it. No super soldier program, no army of Spartans. Oh, and it explicitly state dthat Master Chief was the very last one of them after Reach. This was all retconned after Halo 2
Good luck, and let me know if you have more requests. I didn't play God of War, although I did see some videos, this was before GoW, and it is not "exactly" the same, the feel and flow of the gameplay is less arcadish: