I quite like to play this sort of mini game for about 20 minuets in one go. But, that doesn't mean i wont revisit the game and play for another 20 min at a later date. Let me know how things turn out because i think game play time is a big issue for these 'mini' games.
Godliest
What you want is a game that never gets truly impossible but rather just gets to fucking hard it is for 99,9% of the people impossible yet you can see how someone extremely skilled can beat it. Think if Tetris got impossible after a few "levels", it would've made the game so much worse, now it just gets to a point where most people can't get past but through practicing you can. That's what makes making these "simple" mini-games hard: they have to have a simple way to play them, present the player with an increasing challenge and make the player able to improve and get further through practicing.
Just watch that guy who plays Tetris at fastest speed (you can't even see the fucking blocks until they hit the ground) with them being invisible. It's not impossible, just extremely hard. That's a well-designed simple mini-game.
TheBoob
You know that feeling when like, you know something, but untill someone else says it, you cant put your finger on it. Well i think you have summed it up perfect.
Impossible because of player skills (reaction time, etc) = good
Impossible because the game doesn't allow it = bad
I now have this written down next to my computer monitor, ha