How easy and simple should a good game be?

I had been wondering this question for a long time. I have tried and played many games of different types on different platforms since I was around 7 years old. I played them and enjoyed them, but hardly thought why I liked them. Till high school, when Angry Birds's thunder hit the world, I started to think about why.

Angry Birds was the very first game that made I realize that how simple and easy a game could be with still lots of fun. All you need to play the game is drag the birds on the slingshot, adjust and release. Surprisingly simple, also surprisingly fun. 

Ever since that, I'd been especially interested in trying simple games for quite a while, I was fascinated by the easiness and effortlessness of playing those games. Meanwhile, it seemed that more and more game companies also started to pay attention to create simple fun games. Then Temple Run came as a hit, players around the worlds enjoyed it so much by just swiping on the screen and tilting the device. This time Temple Run brought some more exciting and intense experience which Angry Birds didn't really have.

After that, I downloaded Tiny Wings, I was once again astonished by how simple a good game can be - all you need to do is just to hold your finger on the screen and release at a proper time. This is probably the easiest game I've ever seen.

Consider all these games together, I realized that, as a high school boy, they all have one thing in common - the easiness is only for the players. The levels, the monsters, the power ups, everything behind the easiness are really delicate which seem to need lots of efforts to finish. So a simple but good game could probably be composed of not only extremely easy operation, nearly no learning curve, but also quite complicated and delicate  "backend" design and implementation, as well as a great and creative idea. In other words, to begin with a good idea, do all the heavy works and save all the easiness and effortlessness for the players.