With the voronoi you are simply carving into the terrain. That's all there there really is to it. Since vononoi is a cell like structure it creates a rocky effect. You can cut it as deep as you want. That might be an angle to explore. The problem is that this is quite an extreme effect and you wouldn't see this all over terrains so in comes altitude blend.
The altitude blend can be used in several different ways. You can altitude blend very different terrain forms. That's what we did with the voronoi or you can take two terrains that are very similar to create subtle changes from one to another - this is actually a very useful way to do things. The terrains can have different displacements. It's the fractal's shape that counts most i.e. one terrain could be more displaced than the other or use the same terrain but send it to two displacement nodes. One more displaced than the other. You'll then find that along the join in altitude you will get more sudden change. There is really a whole lot to explore here. Matt gave us the tex coords from xyz. How many people are using it?
Also, and this is important, the more you can build into the initial displacement of the terrain before compute terrain (rather than use surface displacements after compute terrain) the better.
