Popularity is a deciding factor, level of violence is another which also limits the rating.
You will see simillarities bettween the most popular games, they have the most advertising and as such the larger companies have a bigger say.
As to your question, what do you like about the games you play, I can attest to the fact that a game needs to have a gain, whether it be money or abillities, a game needs variety, it should not be lineere.
money is a gain that furthers your gain, A game needs a monetary system, a game needs a sytem of advancment, a game needs ambience,