I actually think it's hard to accurately describe the notion of a hardcore gamer.
My idea of a hardcore gamer is simply someone who treats the hobby as more than a casual pastime. If you sit down and play COD or Fifa for a couple of hours after work/school with your friends, I don't think you fall into the hardcore gamer category.
To me, hardcore gamers are people who take games beyond the gameplay. That might mean that they contribute in discussion forums, read up additional information about the games online or pay more attention to what's going on in the game industry. They wont limit themselves to new releases either. As far as I'm concerned, games are an art form just as much as books or films are, and like book or film enthusiasts, a hardcore gamer shouldn't limit themselves to new releases because the best examples of the art form are quite often the classics.
I guess to put it into fewer words, a hardcore gamer is someone who's passionate about the games they play as oppose to someone who's just playing to kill a few hours.