Casual gamers will generally see hardcore games as needlessly complicated. Hardcore gamers will generally see casual games as overly simple and thus boring. And thus, a divide was born.
Let me give a possible alternative explanation for these terms and the divide from a I-have-no-idea-what-type-of-gamer-I-am sort of a guy.
I think marketing types are to blame for all the overuse of stereotypes and general innovation killing in gaming-land of today.
It's possible that some time ago the people for whose job it is to increase profits probably decided that they were only selling their company's games to boys. Hence they could more than double their profits if they sold equally to girls, and then some more to older folks.
How to sell games though to this new audience with little previous exposure - pretty the games up, make them simpler, and have a "hold-your-hand" button.
The games were made much simpler, at the same time cheap RAM began to be made use of, by adding more and more save features+extra lives - ostensibly so that one didn't have to repeat things ad nauseam. This simplification removed a lot of the sense of satisfaction about beating a new challenge in a game. It also removed a feeling of fine control over the character, as it wasn't really necessary to acquire this over time to complete anything (for both developers and players).
So the games became simpler, more shallow, and less time was spent actually controlling the (now nicely rendered) character/object.
This I think is where the casual gamer/hard core gamer labelling comes in. Some people still wanted games with difficult to obtain objectives and gameplay. So, based on stereotypes of the day, the marketing types decided large amounts of button pressing (where it had to be the right buttons you mashed in order to progress) and enemies with huge health points would satisfy this. I am of course talking about first person shooters of the "hard-core" type. The rest of the games with lower difficulty were to also be sold to girls/older folk, and became "casual" games.
This divide between casual/hard-core seems to have happened over time though. It's as if all the good game developers jumped one-by-one into a big hole at different times, and are slowly trying to climb back towards the light.
I'll at least be happy if this game and others like it bring back that fine sense of control, innovative and varied game design, whilst keeping the possibility to die and have to redo things to get that bigger sense of achievement when completing something. Who cares what it is labelled by the media if the enjoyment you can get out of it is longer-lived, so much that it is really worth your while to play. I want to see more games that aren't about cheap, never ending victories, and actually challenging gameplay made fun.
Sorry if that all reads like a rant. I'm not usually up this early in the morning.