In addition to the elemental affinities of the various locations depicted, and the potential treasures that might be gained from exploring them, I've been thinking of two other ways to differentiate the areas.
Actions: The next method (the third in total) revolves around the types of actions that might be more effective. These actions aren't necessarily easier, but visitors who engage in such actions tend to produce more significant results.
Threats: The final method of differentiation involves the typical threats inherent in each location. These are the types of risk a group is likely to face when they engage in actions here. Like the other differentiation symbols, a specifically indicated threat isn't the only way the characters will be in danger, it's just more common for that type of threat to be significant.
I'm also thinking of shifting the symbols to a more 3d style.