I played some games a while ago. Let me tell you what I thought about them.
Perhaps, my favorite game I've played from the selection, was Don't Shit Your Pants. While the game is simple, it prides itself on one skill alone: Critical Thinking. You might wonder how a simple game can involve so much critical thinking. It involve you selecting the write phrase to proceed trough the game. The game will punish you (and amuse you) for choosing the wrong words.
This Is the Only Level is another favorite of mine. The key ingredient in making this game is derivative design, where the same level has one feature changed just to make it a bit different, yet so challenging. I don't know how many derivations there are in the only level, but the game is quite addicting.
Canabalt was also a simple game, yet challenging. The only thing a player must do is hit one button, the jump button. However, it is a matter of timing. I suppose the key feature of this game is timing, understand when to hit the jump button. As the game progressive, the speed picks up, making it harder.
A very amusing game, QWOP, in which you control the thighs and shins of both legs and attempt to run. I always end up failing, and I guess the whole game is trial and error, and figuring out the physics.
You Have to Burn the Rope was also another interesting game. The game begins by telling you the rules and just lets you go to accomplish one single objective: Burning the rope. It's not even hard, but I suppose the art of it is the experimenting of being clear and concise with rules.