CLIFFS!

[Galacticruler]

basically something like the cliffs in Age of Empires 2, where they were just an impassable object that couldn't be destroyed. all they did was give the appearance of altitude changes. I figure getting them into world-gen would be fairly easy. It'd be an interesting limiting factor to have, perhaps even allow ores to spawn in a cliff and make it a depleatable resource with a special mining method?

There could even be designated cliffs for different terrains.