The debate existed under different names such as "N00bz vs 1337" "Casual vs Core" "Button Mashers vs Advanced" players. The ongoing dilemma between designing games for casual or core audiences with each side adamant games must be designed and developed for a particular group to be successful.. Trying to design for the casual market and you "dumb down" games too much and alienate the core market. Trying to design for the core audience and you alienate the larger casual market by devising mechanics too complex. Casuals may constitute the larger purchasing market, but it's through the reputation of a game from the hardcore that lays the reputation of a franchise and foundation contributes to profitable squeals. The bottom line is you want the game in as many consumer's hands as possible. So why not design and develop for both?

The 3rd pillar of the fundamental truths of game development is Games are for Players. The principle that developed from this understanding is:

Depth should be done in such a way that it's not necessary, but is ideal for maximizing performance. Thus the masses don't have to, but can still "accidentally" access more functionality. Advanced players can apply their skills and purposely access deeper game play mechanics or tactics due to their higher level of understanding or superior hand dexterity. Gears of War and Bayonetta have game play mechanics that are devised for both the casual and core audience.

Gears of War 2: Reload Mechanic:

Each weapon can chamber "X" amounts of ammunition. Once the chamber is empty the weapon needs to be reloaded before being able to fire. Reloading weapons was developed in two ways:

Picture taken from Gears of War Gears 2 gamers blog

Bayonetta: Very Easy Automatic Mode

The reverse of the principle. The combat design is intended to be complex and difficult for the hardcore market. Platinum Games developed an automatic mode for the players that lacks the desire, skill or time to master the hardcore game play mechanics. The depth of controller interface and decision making is available for the Core player, yet it's not necessary for the casual player in order to advance through the game.

Most decisions will be made with the target audience in mind. Designing mechanics so that the game is enjoyable  to as many people as possible is common sense. Design the core mechanics so that the casual has accessibility  in game play but design depth that the hardcore can take advantage of and improve performance. Who cares how the player plays the game, as long as they had fun and bought it new ;)