Also worth mentioning that Badwater Basin creates a great sense of scale by having a lot of empty land that stretches a long way, forming gentle hills (which hides the point at which the land ends and creates nice outlines along the horizon), and that the mountains themselves are right at the edges and are relatively very large.
Also remember to vary the tops of the cliffs by the Z axis to create interesting rises and falls, or to give them a cartoonishly wonky outline!
@Joshenkstone
If you want to look more at these features or see how something else was done in the stock TF2 maps, I recommend taking a look
here. It includes editable .VMFs of all the maps.