I like to think hardcore gamers fall into six categories:
1) Highly competitive gamer (could be professional)
2) Highly obsessive gamer (in a completionist sense)
3) Highly skilled gamer
4) Marathon gamer
5) Masochistic Gamer
6) Dedicated Gamer
1) Highly competitive gamers don't necessarily need to be of high skill, they just need to take a game seriously enough to want to win whenever possible. I don't just mean preferring to win, winning is their one option. Only a small percentage of this type has what it takes to game professionally and many are prone to raging when they have experienced a severe whoopin'.
2) Plays one game for years to uncover any and all secrets. Sometimes, they can be like Masochistic Gamers in that they will do something seemingly arbitrary for the sake of experimentation and will probably examine an internet rumour if it means uncovering something they don't yet know about.
3) Speaks for itself. You know 'good' gamers. These people have the skill of ten of them. You don't know how they got so good. You suspect they may be cheating. Honestly, they scare you a little bit. They are just lucky enough to posses the traits required to thoroughly master a game.
4) Plays a game of Civilization 5 in one sitting... as a warm up. Not opposed to pulling all nighters or "poopsocking". They have way too much free time and it shows. You just hope they don't end up as a news headline at some point in the future.
5) Will play a game on an untested difficulty, despite developer warnings, despite logic, despite the laws of physics. The gamer embodiment of "if there's a will, there's a way". If they can disadvantage themselves in some way, they will do it. Somehow, they always beat the game. This "throws out the challenge" to the next Masochistic gamer to do something even more crippling to one-up his damned brethren.
6) Nothing fancy here, just someone who regularly games. Closest to the definition of hardcore, as in possessing intense loyalty.
Or at least, this is how I see it. I'm sure there are more examples.