I'd like to chime in here.
For me the great thing about Terragen, is that it is almost open architecture.
My background is having a fascination with analogue synthesizers. Where you could connect anything to anything and often come up with shit. What was my breakthrough with sound is understanding how waveforms can be shaped with other inputs.
What's similar here, is that you have something very simple. A black to white fractal as the core of everything.
Lets take thin cracks. I have a black and white fractal. If I offset the same fractal by 10cm in x and z, then subtract that from the original, you end up with an outline which is 10cm thick in x and z. Because you have taken white away from white. 1 - 1 = 0.
What you have to learn is that Terragen will interpret these values in it's unique way. So if I take 0 - 1 as a connection, then I will get -1 as an answer, which is important if I want to connect that to a displacement node, because guess what, the displacement will be negative one meter. If you don't want to have negative values, you either clamp the output, or you turn negative into positive values with the abs node.
An understanding of what each function will do to the chain is extremely useful to be able to shape the chain to what you are trying to achieve. Which leads me on to the next point.
I spend a lot of time thinking how I can create stuff, which gets me into abstract ways on what I need to do to get there. So you have to have a train of thought to apply those blue nodes. Connecting shit to shit often ends up with shit.
It really helps to have a mental visual of what you think should happen, then bash that against the networks to see if you are right. A lot of times you are wrong. But when you get it right, you've often just built something nobody else has made. And that's bloody awesome.
Blues are the fundamental building blocks of all the red shaders. Matt has put a few together to make life simple. But no matter how good Matt is, he can't think of every scenes requirement and has given everyone the opportunity to build there own solution.
I really like that.