Without trapping ourselves by making our opponent's argument -- after all, the hallmark of the casual game is that it is easy to pick up and learn -- we can instead focus on why it is that hardcore games do not attract that market, and what it is that casual games offer instead. To sum it up in a word, it's depth. But wait, isn't depth what people usually say these games lack? To answer that, we need to revisit what "depth" in games really means.
When pundits talk about game mechanics, they like to toss around the term "depth" to apply to play that offers a rich, engaging experience on multiple fronts. Take the aforementioned Starcraft, for example. This game presents players with an almost bewildering array of gameplay options. To begin with, there is a hierarchy of units to consider. You can choose from one of three races (Terran, Zerg, and Protoss), and each has their own unique tree associated with them, consisting of no less than at least 7 identifiable units per race. Within the units themselves reside a small number of unique abilities.
Furthermore, there is a nearly overwhelming range of strategy choices to be made at any one turn, and each choice is equally important in the continuum of the overarching campaign. This dizzying array of decision paths is one of the reasons why even now, after the 10 years since its release, Starcraft continues to remain at the forefront of tournament play around the globe. It's as fascinating to watch as it is to play, and rewards those who take the time to invest concentration into it. There is definitely nothing casual about the game.
However, it's a mistake to think of Starcraft as being a "deep" game. To illustrate this point, let's imagine a horizontal plane, with each race and their respective units and the myriad strategic ploys arrayed along it. We have a fairly broad plane indeed, but almost no depth. Each of these choices, from unit deployment to unit ability activation, to the larger dance of group movement and ground control, is merely a point on that line. Each serves its one function, and once that function has completed itself, it's finished.
What makes the distinction here is balance. Each Starcraft unit is so well-balanced that a player chooses a particular race only out of personal preference, perhaps for that race's aesthetic. Each race's units have their opposing and equal units represented in the other two races. There is no one unit that drives victory and thus, no heavy, conceptual point, drilling down into game space. Think instead of Starcraft as being a "broad" game, where each element widens the concept of the game, and over which any number of mental constructs can be laid. The Starcraft universe is broad enough to give rise to a wealth of narratives, from the official stories Blizzard creates, to the smaller, more personal and intimate tales conducted on an almost daily basis by players around the world. This cannot be done with a game like Zuma, or Chuzzle.
Take, instead, the mechanics of a game like Epsilon. Here, gameplay centers around the manipulation of two "wormholes", each one the entry and exit of the other, Portal-like. The object of the game is to guide an orb into a series of tokens and thence to the exit, thus completing the level. And that's all there is to it. Certainly, there are additional concerns as gameplay progresses, but these gameplay additions serve only to further intensify the pursuit of the goal, which is to collect tokens and guide the orb to the exit. If you envision the game mechanic as a shovel, with the ultimate goal being the completion of the game, then each level can be likened to a scooping out of dirt. Each scoop deepens the existing hole, and thus true depth is revealed.
There is even a temporal element involved in casual games that further enables the feeling of sinking into the depths of play. After all, who hasn't sunk more hours than expected into something like Zuma, only to suddenly look up to see the time, and suddenly feel as though sitting at the bottom of a deep pit?
At some level, of course, this is mere conceptual jiggery -- the ideas are the same, regardless of their particular nomenclature. Yet what goes on during the playing of a casual game does deserve more respect than the hardcore ascribe. There is a transformative, almost alchemical event that occurs once you've played past a given point of Tetris, for example, where the action ratchets up to a nearly unbearable level. In the back of your mind, you know that the game is unbeatable; it was designed not to have an endpoint. Yet you soldier on, trying to complete that one last row before it all piles up on you. The intensity of that experience more than rivals that of any marathon session of Starcraft, if only in a different way.
The mental toughness and stamina such casual games require of their players is matched on the hardcore side by mental agility and inventiveness. The ability to concentrate energy into a point is the equivalent of the ability to expand energy to a plane. Once we've evolved past the point to need categories, perhaps we will be able to develop games that include both the depth and breadth inherent in today's segregated game genres and really have something amazing on our hands.