A 'hard core' gamer, is a person that can find a justification in playing a game when no one else could.
The game need not be great, it need only be something the hard core gamer couldn't resist playing. Think smoking, can you walk away from a game? Then you are not hard core.
I have a Nintendo DSi XL, it is in excellent condition, because I don't play it to death. The battery will likely die from old age before being worn out. I have copies of a zillion DS games I might never get to, titles most would consider must play titles. But the day is only so long, and there are so many thing to do other than Nintendo games (believe it or not). I am sure not a hard core gamer inasmuch as Nintendo DS games goes.
But, you should see my collection of PC wargames. I have bought games worth 80-90 bucks and have yet to get around to playing, titles that have expansions that effectively make the sum total price around 150 bucks. That's a lot of cash for games I hardly play. So am I a hard core wargamer or not? Hard to tell. I do other things besides wargames.
I think the term that might apply to me is merely addict
I can't stop buying wargames. I even buy some mainstream titles just on the idiotic belief 'I might get around to playing them eventually'. That's why I bought Dragon Age for the PS3 and why I bought Guild Wars 2 for PC. Likely money wasted (it's wasted money if you don't play the game regardless of how everyone says it's great).
I don't think you are hard core if you play the game to the detriment of your health. I believe that is merely called stupid.
I have friends that own all the consoles and play all the top games and play them a lot in their spare time, but, they still have jobs and they still have things like housework. They have other interests as well. But they play their games 'hard core' if they can claim to have 'beaten' a new title in just a week.
I think a hard core gamer is someone that can state they have all the consoles regardless of whether the consensus states some of the machines suck for one reason or another. If you use Steam even though you think it sucks, and if you are ready to get an X Box One regardless of all the brain dead spy crap you are likely hard core. If you consider a specific version of the PS3 a vital item for just one aspect of the machine, you are likely hard core. I have a PS3 for instance and it sits on a shelf unused currently. I don't even care what flavour or machine it is. I am not that hard core about it.
I think the only thing I am hard core about, where games are concerned, is I am not interested in the opinions of adults on what is and is not a suitable game for someone my age. But then again, like the sig says, I ditched my age group, they suck when it comes to thinking growing up is worth something.