Many players get frustrated when their lower level friends teleport to them while they are in battle. I had the idea to give level requirement to enter a world: WizardCity - Open to everyone, obviously. Krokotopia - Open to level 8+ Marleybone - Open to level 18+ Mooshu - Open to level 28+ Dragonspyre - Open to level 38+ Celestia - Open to level 48+ Those requirements are fairly low, and would let only persons that could be helpful in that world to get there.
Of course, you don't need the level requirement to enter the world if you have the spiral key. But anyone without access to that world should have those level limits.