Urban Dictionary FTW!!!
Hardcore Gamer
One who spends in excess of more than 4 hours in one sitting. Realises that graphics, story, fun and all the other shit are all equally important. One offsetting the other will affect the game adversely
Many people claim to be hardcore gamers, but they are just posers. as stated above, all of the mentioned above are needed equally in order to make a kickass game.
My own def since Urban Dictionary FAILs
Hardcore game
A game with a perfect balance of graphics, gameplay and sometimes story and multiplayer gameplay that drags a gamer into the world. The game drags a gamer into the virtual world where they feel the emotions and/or feel that they are apart of the adventure of the characters they play. For some games, like World of Warcraft or Halo, the depth of the online experience drags players into the game through the need to improve yourself ability to pay better and become as close to the best that you can.
The games don't have to be AAA titles, they just have to give an experience that engulfs the payer in a fantasy land.