wtf, "hardcore gamer" only makes me think of internet tough guys, but those usually make me lul, thus really failing at being "hardcore".

Ever heard of a "hardcore tv-watcher"? No. Hardcore per definition claims that there is some kind of skill/talent within those that take games way too seriously.
Only thing I can think of that is appropriate, is that "hardcore" gamers don't learn any longer (aka knowing everything worthwhile about the game) as opposed to causal gamers (DS target audience, aka your grandmother).