There's degrees to this. It's fine to have to concentrate and learn skills, but the moment I feel like I am *working* instead of playing a game, I drop it.

A game's difficulty should only be only so high as the player can win while staying focused for short periods of time. Skills to win the challenges should take into account what he may have learned this way previously.

It's a balance game designers should observe. Any need for "grinding" is a failure in design.