Exactly. I would add though that you also can determine how hardcore a game is by seeing how easy it is to pick up and play. The boggest franchises like Mario and Pokemon are generally casual, because they're easy to get into and appeal to a wide variety of people. More obscure franchises like Earthbound and Fire Emblem - while popular among a lot of Nintendo fans - are far more challenging and time consuming, which is why they're hardcore. There's probably not that many 10 year olds today that would find Ness and his quirky friends as appealing as a game of Mario Baseball or Mario Kart - and most probably haven't even heard of him apart from Smash Brothers.
Ironically, this would make the Metroid Prime series far more hardcore than Halo, despite a lot of fans seeing it the other way around. Despite the wonderful reviews Samus's 3D games have gotten, they still haven't sold nearly as many units as Halo. Halo is actually pretty casual compared to a lot of other series as well - online play is quick and easy to get into, and the story mode isn't the main focus. The reason why people call it hardcore stems from this silly misconception that "hardcore" means "dark, gritty, and realistic." Gamers 15 years ago would probably be laughing their controlers out of their hands if they saw the current state of the market. :P