Regarding human figures, I think part of the problem really is in Terragen. I've seen human faces modeled in Blender that were so life-like that I did a double take. But when imported into Terragen, they loose much of the realism. Probably part of the problem is that TG3 doesn't offer some things like Sub-Surface Scattering (this SSS has nothing to do with Terrragen's SSS - Simple Shape Shader). Terragen is brilliant at some things, but not good at others. Like falling water. Mix the uber-realism of sky, rocks and ground with starkly unconvincing artificiality of a Terragen waterfall, and you got an image where the bad parts are jarringly noticeable.