A difficult question indeed. Some game can be both "casual" and "hardcore" at once. I think I'll get back to this thread later with some more thoughts.

Depending on the playstyle, pretty much every game can be casual and hardcore. However certain games/gaming series represent a certain playstyle that is mostly considered to be either casual or core and thus is played by the majority of their owners in that way.

To me multiplayer only games are already a huge indication for a casual game. Some of them can surely be played in singleplayer as well, but they have no difference except that your playing against nobody at all or the AI. Examples: FIFA, Mario Kart, Just Dance.
In other words, no focus on story. Also when the main focus lies on kids or non-gamer, thus simpler gameplay.

An indication of a core game is surely the difficulty of a game (even more so if it's set, so you can't change it). Also more depth in gameplay, less accessible when you made a longer break or a quick gaming session.

Of course there are also games that have a mixture of both, so you can't really say this or that.

The Industry definition of the casual games genre includes games that focus on brain teasers and short play sessions: Minesweeper, MS Solitaire, Frogger, Freecell, Farmville, and Brain Age. The genre was coined by Namco when it released Pacman back in 1980, and later re-iterated by Nintendo to classify Brain Age.

Mario is a platformer, Zelda is an action adventure, Mario Kart is a racer, Pokemon is a light RPG/console RPG, Monster Hunter is an action game. None of those are in the casual games genre.

There is no such thing as a "hardcore game". That's mostly just a term that a portion of gaming fans use to make the games they play sound "badass" (and not just fans of bloody/violent games either, but also Mario and Zelda fans); the genre doesn't actually exist - and the term is really just pretentious.

In fact, most of the time when I hear the term "hardcore", it is used by very nerdy people to describe their hobbies - especially the music they listen to and the games they play. The term "Hardciore" is used in an attempt to artificially elevate their hobbies; and in extension, elevate themselves to a socially higher position than where they actually truly exist.

While i will make a formal objection to the notion of hardcore and casual, i would say that anyone who considers Monster Hunter casual has never farmed for Wyvern Stones, or felt that surge of fear when Deviljho interferes with your first high-ranked quest.

Their is no such thing as an casual/hardcore game only casual/hardcore gamers. When devs/journalists talk about game X being an hardcore game it means that it is an game made in the first place for the hardcore gamers..

All right, here's my take:

Mario - Casual, as of recent
Zelda - Casual, as of recent
Halo - Hardcore
Grand Theft Auto - Hardcore
Call of Duty - Hardcore
Mario Kart - Hardcore
Madden - Casual
Uncharted - Casual
Street Fighter - Hardcore
Final Fantasy - Hardcore
Monster Hunter - Hardcore
Smash Bros - Hardcore
Guitar Hero/Rock Band - Hardcore
Gears of War - Hardcore

I still hate this entire debate and use of the word "hardcore" though.

Well I think what gives rise to the debate is its use in different contexts, we could be talking about either;-

1. Difficulty
2. Appeal
3. Average play session

A lot of the Nintendo titles cross over in these areas. Games like Mario and Zelda consistantly garner great reviews, which mean they are coveted by the enthusiasts and their clean image and reasonably easy difficulty make them accessible for the mainstream.

Whatever the frat boys are playing this generation.

a vaque definition.

Brilliant :)