For me I suppose I'd say hardcore = competitive. The kind of gamer who has played a game so much that they're able to filter out all the cosmetic "fluff" and engage directly with the "crunch" of the game mechanics. Examples would include high-level competitive FPS players, Streetfighter players, the kind of person who can get a triple-A rating on the hardest mode of Dance Dance Revolution, people who do speed-runs of games, and so on.
Basically, the type of gamer who doesn't just play frequently or passionately or even compulsively, like achievement hunters. I'd call a hardcore gamer somebody who approaches a game with the mindset that they're not playing it, but working on it. Sharpening their muscle memory, finding optimal strategies, seeking out and abusing bugs and glitches to the point where they've dissected the game and have an intuitive mastery of it.
I occasionally see this kind of gamer while playing Left 4 Dead 2 and it's insane how good they are. They have an encyclopedic knowledge of each level, know when crescendo events are coming and where the optimal stand-off points are, abuse the check-point and regeneration mechanics, and are intimately knowledgeable about how to combat each type of zombie. They don't panic or get caught up in the "fluff" of the game, I can only assume they view the game dispassionately in terms of Matrix-code and treat the whole thing as a dynamic equation that can be solved. It's simultaneously impressive and a bit worrying. They don't just know the levels and have the ability to shoot straight, it's the mindset that separates them from other players.
So, yeah, I don't think any of the following alone can make you "hardcore":
- Having played games since you were X years old
- Having played games for Y years
- Being so into the canon or culture that you get gaming tattoos, name your kids Zelda and Master Chief, etc
- Having a huge game collection
- Having a huge Gamerscore / trophy collection
- Playing for Z hours every day
These would make you a fanatic, or a collector, or a buff, or possibly a bit of an obsessive. A true hardcore player takes things to a different level.