As blattsalat said: erotic, sexyness are very simple things to make a good game.
They are natural drives which makes the brain say "play it on until you found everything". This works also with collectable beautyful things and dominating (all the natural drives).
This do not realy work with games for girls and women.
I think erotic is the best of all drives. It makes the player the most wanting more, next to dominating (fighting games, 3D shooters). Collecting can be very good. And one more is knowledge. This is the most difficult. I know only three games which I realy recomment for learning something about life: SilentHill2, FinalFantasyVI(III) and VampireTheMasqueradeBloodlines.
SilentHill2 - is the ultimate idea of all ideas. Letting the player feel what the "hero" is feeling by interpretating the world.
FinalFantasyVI - no, the hero is not the most important of a game as final fantasy VI shows. If someone knows the aniamted batman series. Some of the episodes were good but some were bad. Why? They are all batman! Because it´s the bad guy whos important. He must be intelligent, must be able to defeat complete legions without super powers, he must be a genius, the player "must" understand/feel with the enemy, and the bad guy must be interesting!
VampireBloodlins - Again the bad guy thing, but even more. You can be a taxi driver and defeat a complete "religion/state/kingdom", if you know how. I won´t tell the story of bloodlines here but it´s fantastic, logic and "who don´t know it missed something".
As I said erotic is something real good for a game. For example, make a real simple game like arcanoid. After every level the player gets a more sexy pic. And ready is a game with a reason to play every level without getting bored too fast. I wouldn´t have bought street fighter, VampireBloodlines and all my other games without all these sexy girls.
And after you have a good story/idea/sexy pics you can make the game as good as you like. You learn creating good games, by creating them (don´t stop! Then your stamina and knowledge will raise and your games will get better and nothing will be impossible anymore but you will see what´s needed and what is not) and by watching how others do good games (for example bloodlines: the levels constist of MODELS!). I hope I didn´t forget anything. But I am sure I did. But the most important things are told.