Simple mechanics. Shallow learning curve. Frustratingly hard challenges.
The Impossible Game (a 360 Indie Game) is not the kind of game I expected to like. At all. I do like a good challenge, but don't like frustrating/maddening difficulty -- especially when a major source of "challenge" is poor usability of controls and/or game play feedback.
The use of compelling music, simple controls and patterns, instant respawns after failure (with no penalty in terms of loading time or excessive backtracking required) and a great "quicksave" option meant that I was quickly hooked and converted.
The game inspired me to keep taking on more and more difficult challenges in a way that few other games have done. A couple of the games I reviewed for the IGF competition shared similar characteristics -- they embraced player failure and made it a feature of the game. Failure was treated as a learning and teaching opportunity, not as a punishment that left me wanting to quit.
I highly recommend checking out the trial mode for yourself.