Ok, so I'll tell you all this. The terrain is important, but what matters the most for my terrain maps is the lighting, fog and visibility. For instance, on Fiery Isles, the fog allows for 3 advantages.
1) Better view underwater.
2) Ability to see faraway islands more like shady sillouettes.
3) Allows spacing effect between sky and clouds.
Rusty Desert's and lighting simulates the time at about 4:00 in the afternoon. The fog and visibility distance makes the air seem dry and seem like there's a weak sandstorm.
I usually spend a good half-hour just on these effects in order to make your expiriences more realistic.