There is no such thing as a "casual game". Firstly, I want to discuss what we think the word means to begin with. Wikipedia's page for Casual Game says:
"A casual game is a video game or online game targeted at or used by a mass audience of casual gamers."
Well, firstly, what game wouldn't like a mass audience? There are probably some, but they are so few that it's basically negligible. Also this doesn't seem to separate it from what people consider "hardcore" games - are XBox FPS games going for a small audience? And the second part, "audience of casual gamers" is just using the word in the definition, so that doesn't help us.
"Casual games can have any type of gameplay, and fit in any genre."
Still no information...
"They are typically distinguished by their simple rules and lack of commitment required in contrast to more complex hardcore games."
Here's where we get to the crux of my argument. Simple rules? Lack of commitment? I think this statement is completely false. "Simple rules" is a design goal for any game - you always want your game to be as simple as it can possibly be for whatever you want to express... sometimes this may end up being somewhat complex, but you still strive for simplicity as much as you can. An example is Civilization V cutting out the religion mechanic, or Team Fortress 2 cutting out the armor mechanic - whether or not you agree with these decisions, they were made in an effort to keep it simple. Some of the greatest, and most hardcore games ever made have some of the simplest rules you will ever find. Look no further than the game "Go" for a prime example of this. It doesn't get any more deep and competitive, and it doesn't get any more simple than "Go". So simple rules is not something we can use to define "casual games".
How about this "lack of commitment" thing - it means that you don't have to invest a lot of time, money, or effort in order to get playing. Again, isn't this a design goal of ALL games? What developer would ever be like, "Well, we want players to have to jump through a bunch of unnecessary hoops before they can really start to enjoy themselves". You may be thinking "I feel like a LOT of developers do that" - but they never intentionally make their games boring. If you play a game and feel like it's making you jump through hoops before you can get to the fun part, then that is simply a poorly designed game. A well designed game should be easy for all players to get into. "Easy to learn, difficult to master" is the mantra. No game developer intentionally places obstacles between his players and his game. Publishers and hardware developers - they do create obstacles in video game consoles, but their goal is never to create obstacles, their goal is to make a good platform that will make them money.
Maybe your argument is "well, a casual game has only a few buttons you have to press, whereas a hardcore game has like 20 or more different buttons and combos that you have to learn." My counter to this would be that those games with 20 or more button combinations are more than likely not as well designed as they could be. I am a huge roguelike fan, for instance, but could the interface for Nethack be improved upon? Hell yes it could. Dwarf Fortress also comes to mind!
Another popular idea of what defines a casual game is difficulty - specifically, lack of difficulty. It is said that a casual game should be easy, whereas a hardcore game should be hard. I believe that both of these are incorrect. Again, the goal is to make a good game, and a well designed game has what I call "balanced difficulty". All games, in order to be fun, require a good level of challenge. Without the real, honest threat of failure looming, there is nothing to create tension, and nothing for the player to feel good about once he has mastered the skill required to overcome the challenge. In short, all games need to be at least "somewhat hard" - I'm not any more a proponent of brick-wall difficulty than I am of Kirby's Epic Yarn, in which failure is completely impossible.
In short, we need to get rid of this idea that there are "casual" and "hardcore" games. We need to shoot for something that everyone who likes games at all will like: good games. Tetris, Super Mario Brothers, Chess, Soccer - these are all both "casual games" and a "hardcore games". And that's what we should all be aspiring to.