Games are games. There is no such thing as a casual or hardcore gamer.
Were we calling Duckhunt, Pong, and Super Mario Bros. 3 casual games when we played them in the 1980s and 1990s? No, they were good games gamers bought.
Is it a matter of time spent playing? Well, if my girlfriend plays Wii Sports for 4 to 5 hours each day, then is she a casual gamer because she actually has a life that prevents her from holing herself up in her room for 8 plus hours?
Is it a matter of content? You would have a point here, but lets consider the converse. If a game such as Wii Sports which has sold more than 25 million on a single console is to be considered a casual game, then how do you argue that those 3 million playing Assassin's Creed 2 are hardcore gamers?
I would argue that Wii Sports is more of a hardcore game because it has more people playing it, whereas the hardcore gamers are your outliers who want to be unique and play games only they play. What is so hardcore about playing a game that has a fraction of the gamerbase than a Wii Sports?
This whole casual vs. hardcore is ridiculous. It is like telling a person who plays Tetris that they are a casual gamer because they are not playing the games you play 24/7 because they prefer games that do not consume their life.
Hardcore gamers are the outliers and any business will kill itself catering to a bunch of fickle, rebellious brats who think they and 2 million other players are unique. I would argue the games with the larger the player base are the hardcore games, while your Assassin's Creed 2 and other game's player base is casual because they are not as numerous and they are extremely selective to the point where they only buy certain games at the expense of having fun with all of them regardless of genre.
A hardcore gamer is someone who plays it all, while the casual gamer is like a teenage high schooler constantly looking to make a fool of himself by shunning what is mainstream and trying to niche himself into obscurity.
