I'm going to concentrate on the 'casual vs hard-core' aspects of your question.
There are several schools of thought on how to design games in general. The 'compulsion-based' one models 'fun' around compulsions.
You can think about compulsions like 'mini-goals' that our brains have. When these goals are satisfied, the brain pleasure centers get activated.
They are not the same for every one, but they tend to be equal per demographic group.
The 'hard-core' games you mention tend to favor one 2 specific types of compulsion:
- The empowering compulsion, in which you get a system that is gradually better (the player levelling up, getting new weapons, etc)
- The mastery compulsion, which creates fun by allowing the player to gradually be better at handling a complex system (statistics, etc)
The empowering compulsion has a tendency to be stronger on males than females. The mastery compulsion usually appeals more to engineers (hence, also males in general).
The 'casual' games focus on a different set of compulsions. Pet society, for example, relies on the 'accumulation' compulsion. Although I get a certain degree of pleasure by accumulating stuff (achievements in games), my girlfriend gets hooked on that pulsion much more than me.
The Wii games tend to favor the social compulsion - relate with others while gaming.
You feel that these games are too simple because you are very used to getting your pleasure from the mastery or empowering compulsions, and these games provide very little on that front.
Another aspect is the 'cute' factor. I would not call it a compulsion, but a necessary design decision. The vast majority of people will like your game if it's 'cute', or will not mind. On the other hand, if you make a game 'serious looking' then a great percentage of people (nearly all casual gamers) will not feel appalled by it. And that's why nearly all casual games tend to play on the 'cute' card.
EDIT1: pulsions are called "compulsions" in proper English.
EDIT2: This article in Gamasutra explains compulsions much better than me.