Surprisingly often people have a ready game but it's not challenging. It's not interesting, it gets very quickly repetitive and it's so easy to do what you need to do. Or it's too hard, way too hard, you take pride being able to design a hard game. Well congrats, you just alienated your potential players and customers.
A game without a challenge, is like a Dvd player. You just press play. Another way to check who well your challenge works, is to check do you always get the same amount of points from the levels (unless it's more a rpg game).
So provide a decent challenge that matches your target groups learning curve. The challenge you integrate to your game needs to match the players, your target group.
Adjusting the challenge, we can target different player groups and ages. The speed element creates a pressure element. In the 1010 version of the game, the time element is removed, which creates a more relaxed game play style which usually is more suitable for puzzle games. If we increase the speed we can create a different kind of experience and challenge. If we'd add items to the levels we'd need to avoid hitting, we'd yet increase the challenge.
There's a lot of room to create variation with the Challenge.
How would you variate it, if you would have create Tetris that would different to all other Tetris games?