I believe that a game is only hardcore if it has the ability to put immense dedication and strategy to play. Games aren't "hardcore", a player is.
Take Wii Bowling for example. If you wanted to, you could break it down and see exactly how you needed to move the controller to get a strike every time.
It might take a bit of focus and determination, but it's pretty possible. It's just not usually done. No one needs to do that. With regular practice, you could probably get it down anyway, or at least close.
Now take Street Fighter 4. You bought the FightStick, and unlocked every character. Now you get out your calculator and ruler, and start measuring the length of the health bar, and measuring which combos do how many inches/cenimeters/millimeters of damage, then finding the percent of the total health bar it does. You measure the accuracy of the attack, the distance it travels, how fast the attack is performed, the amount of Super/Ultra the attack fills or empties, how fast you can dish the combo out on the FightStick, how much damage it does if blocked, and so on. Then recording how common each character is used, and doing the above process for them first, then learning how to block, dodge, and counterattack that character's combos.
As you can see, if you wanted to, you could do quite a bit of research and study into Street Fighter 4 so you can be one of the most irrationally good players out there. Sure, you could practice and learn how to on the way, but this process could do it in a way much more efficient and effective, even if it's harder to do. THIS is taking a game to a hardcore level.
Now I do realize that I went a bit overboard on that Street Fighter 4 thing, but I'm making a general idea. A hardcore game is a game that can be researched to an extent. Even so, the game isn't just 'hardcore'. The PLAYER has to be hardcore. I play my share of Team Fortress 2, but it doesn't mean I've figured out the most effective places to put my sticky bombs, or the best place to snipe from. I play Team Fortress 2 semi-casually. I play it to relax. I don't play it to be the most ridiculously best player.
Even if you don't do all this research on a game, it doesn't mean that you aren't a hardcore gamer. A casual gamer is simply someone who plays a game every now and then. If you buy game merchendise, or learned the lingo, or participate in tournaments, or anything more that's than the occasional game (maybe with a friend), then you probably are a hardcore gamer.
Most of the people who play online ARE more than just casual gamers. It's pretty simple. It shouldn't be very hard to find out if you are a hardcore gamer or not. That's my two cents.