IMO hardcore is a word to describe how one plays a game, rather than a description of the game itself.
For example, many might consider my Mum playing Tetris a 'casual gamer', but she played Tetris on my gameboy colour for 40 mins each day, almost without fail for 9 years (last year I bought her Tetris DS to play on my old DSlite). My Mum is a hardcore Tetris player, but many other might not be.