I watched a video a little a while ago about the 'one mechanic per world' or rather one mechanic per level thing, specifically in the newer 3D Mario games. 3D Land, 3D World, etc. If it's of interest to you, here's a
to said video. Pretty cool video game design analysis.
[quote:k3xshiir]1. Each level begins by starting the player with the new mechanic in a danger-free environment (such as introducing wall-jumping with no bottomless pits)
2. Next, a portion of the level develops the mechanic, usually by removing the player's so-called 'safety net' and introducing a risk with the mechanic
3. Then a twist is introduced, and either will challenge the player with the mechanic or provide a difference angle to make the player examine the mechanic in a new fresh way
4. Then a conclusion is reached by the end, where the player is given the chance to mess around with one final time with the mechanic, usually by showing their mastery of the mechanic or performing a trick/puzzle at the end with it (i.e, a simple 'puzzle' to earn a 1up or something otherwise)