Im going to try to give a serious and academic answer to a rather not serious question and un-academic question.
A hardcore gamer is a person who takes the patterns, mechanics, and heuristics to a game, and apply's them to situations outside of the normal game playing time and space.
For this reason, people who think about there games in a critical way, and compare and contrast a games content to other games in a critical analysis, dissect a games realism and sim-like qualities, or in some way get involved in the Meta game of a game, are hardcore. Those who who bring the actual game itself into places and times where normal life SHOULD be are arguably hardcore, but also are arguably just obsessive. Those who truly internalize a games patterns, and either construct a larger meaning applicable outside of the game itself (particularly with, "Art Games") are hardcore, and those who attempt to understand a games patterns outside of the context of a game itself, for the purpose of getting better at the game overall, as opposed to a method of just circumventing the acquisition and internalization of a pattern required to play the game casually (Going to the walkthrough in order to cheat) are hardcore. This is why complex games, filled with a large number of intersecting patterns, are considered more hardcore, while casual games that allow the player to exploit patterns without really internalizing them are considered less hardcore. It is also why players of a variety of genres, as well as more unusual and innovative games, and therefor accessing more diverse patterns, are considered more hardcore then those who limit themselves to a particular genre or 2.
