It's well-known that fractal-type objects can be quite lifelike and easy to generate. I've been scripting Minecraft with Python, in preparation for teaching this to gifted middle- and high-schoolers this summer, and wrote a simple 3D turtle graphics class with pitch/yaw/roll support. Like many kids of my generation, I did 2D turtle graphics programming with LOGO in school, but a 3D turtle just has a load of new possibilities. In particular, the 3D turtle allows for nice 3D fractal generation.
Instructions on how to do this stuff in Minecraft are in my Python coding for Minecraft instructable.
It was very easy to generate the following fairly lifelike tree with a simple bit of recursive code and some randomness.
An L-system does a pretty lifelike job even without randomness (using rules from geeky.blogger):
There is something glorious about a world where structures are mirrored on multiple levels. It makes the different parts and levels of the world be like a work of art, with themes and intertextuality.