My general observation on Casual, Social, and Facebook games is that they are generally free to play and have some sort of micro-transactions as a base. My opinion is that this is a sometimes shady practice due to some games creating paywalls that force the player to pay for ludicrous things. For example, one phone game that I am playing requires the player to pay up to $80 in micro-transactions for 30 random characters. Which could be all completely terrible pulls.
I think that a good game that fits this allows the player to play the game and entices the player to buy these smaller things because they enjoy the game instead of forcing them to so they can continue. The other thing that I think that makes a good casual game is that the player should feel the ability to play the game whenever for short easy play sessions without the requirement of a long play time.
I think that the best examples of this genre are games such as game that you are able to pay a set price for and have the full games experience without the micro transactions. Games such as Battle Cats and Cut the Rope are far better due to this because the player has more control over their experience. Another example would be Kingdom Hearts for the phone as it has optional microtransations that really do feel optional. They don’t stop the player and they even have events that help people catch up to their friends in ranks and play the campaign faster. I think that this is another modest way of creating a casual game.
I think the worst examples are ones such as Yokai Watch wibble wobble (A Game I'm Currently Playing) because the prices are far too high. I personally find $5 per random character as a very steep price. When spending that much could unlock anything from a rare to an ultra-common there is absolutely no point in doing so. Other games like Clash of Clans forces you to buy micro transactions if you ever want to stand equal to the people around you too.
I'm busy working on my blog posts. Watch this space!