Well, for starters, they always use a space after using a comma. That's the number one criteria involved.
Then, and only then, once you've got that down, a hardcore gamer is someone who is quite absorbed by the hobby, practically living for it, but also those who breach past the mainstream titles. If you play CoD obsessively, but that's the only game you play, you're a hardcore CoD player, certainly, but a hardcore gamer? I'm not so sure on that. Mind you, I've no hard feelings towards casual players. Even I'm casual in many regards. I have a lot of games, but I go long stretches sometimes without playing any.