Challenge should vary game to game, depending on the intended audience, but it should still hit a minimum sweet-spot where the player is pushed to execute techniques the game just taught. Games are considered too easy these days by players who remember struggling against challenging bosses, confusing puzzles, and annoying enemies then finally feeling the edifying triumph of overcoming adversity armed with wits and skill alone (along with a touch of luck sometimes). It's important to have that in your game so players feel like they are doing something that matters and that they're learning new skills. It takes a bit of work to make sure the challenge is genuine though, and not implemented in a way that makes the players feel cheated. Losing should always clearly be the player's fault, that way the player is more willing to keep trying. A game you work for is a game you'll care about.