@Ian Sane.
I seriously disagree with your statement. What is casual about Super Mario Galaxy or New Super Mario bros. for that matter? Only Mario I can call casual is Mario Kart, and that's mostly made by Hideki Konno. Hell, StarFox is diffucult as hell.
And Zelda is dark(Twilight princess, Majora's Mask, OTT ), serious, has a deeper story than Metroid(God that timeline is complex) and most importantly, hardcore. It's just that some of the graphics are simlified which makes some of the games look kiddy at first glance.
Nintendo games just aren't needlessly violent or over dosed with melodrama. They just try to appeal to a wide demographic.