I don't really feel that most games can simply be "defined" as casual or hardcore. I'll participate in your side experiment, but with a twist to represent my feelings on the matter.
A rating, 1-5, with 1 being most casual and 5 being most hardcore, will be given to each series.
Mario (platformers) - 3
Zelda - 4
Halo - 3
Grand Theft Auto - 3
Call of Duty - 3
Mario Kart - 2
Madden - 2
Uncharted - 4
Street Fighter - 4
Final Fantasy - 3
Monster Hunter -4
Smash Bros. - 3
Guitar Hero/Rock Band - 2
Gears of War - 3
As you can see, no 1s or 5s were given. "Carnival Games" would be a 1. Dungeon Crawl Stone Soup would be a 5(seriously hard game - Demon's Souls got nothin on it). Most games find themselves somewhere in between.