Now, this is only my view of the terms hardcore and casual...
It's only Nintendo-hard if you have to sink 10x the amount of time it takes to play through the game the first time, due to the difficulty. If platform games are that hard, they're hardcore. Maximo: Ghosts to Glory would be a good example of that.
If the game requires over 30 hours to beat in a standard run (including side-quests), it's generally considered hardcore, too. Extreme example: the Disgaea games.
If the game has levels that can be beaten in 15 minutes or less on average, even if it has 100+ levels, it's probably a casual game, because you can pick it up, play it for a bit, and put it away, as a little fun distraction between your important stuff, instead of having to set aside a more significant amount of time to play.
Interestingly, this means that many FPSes out there could qualify, by my definitions, as casual games. Especially the multiplayer stuff. And those are considered by most to be hardcore games.