As an old-school gamer, hardcore is niche games that are many times hard as hell. Or gamer's games. Games not made for sales or pitching to the broadest market. Okami, Geometry Wars, Ninja Gaiden, Murasama: The Demon Blade, Dragon Quest are all hardcore in my old-school opinion! Phantom Dust, Otogi, and Panzer Dragoon Orta on Xbox were definitely hardcore. But you never hear these kids champion those games. Why? Cause they never played them cause they were not meathead shooters. And Microsoft, well they don't support anything like that anymore.
Call of Duty, GTA, and Halo are not hardcore. They are mainstream games aimed at a lowest-common-demoninator broad market. I think Microsoft did a good job changing what the term used to mean. Cause now you hear every lil tyke with a 360 saying that!
I think the next game that actually looks like a hardcore gamer's game is Murasama: The Demon Blade. No it won't sell well, but it will likely be worshipped like Okami is by the discerning gamer.