- from what i can see your scene doesn't need supersample prepass. that setting is usually used when you have a bunch of very small objects in shadows. you should check the sticky thread about render settings.
-atmosphere settings: you usually don't need anything above 64 except in very certain renders. also ray traced shadows on the atmosphere is unnecessary unless you want to get a godray effect. Also when it is behind ground i believe, though that was a bug that they were looking to fix and that was a simple workaround it could be fixed now. also your haze is set high.
- same as above on the ray trace issue. you only need cloud ray traces if the cloud will be leaving shadows on other clouds or something like that.
Those things above will dramatically improve your render speeds, but it still seems slow so i could've missed some settings that shouldn't be touched in most images as its taking forever to render when it shouldn't. In general though, if you don't know what it does, play with it, but put it back to normal if after a test render it had no effect or a negative one.
Also,
-you should try using some free objects. check out the xfrog and walli free packs and learn how to place some trees/grass etc to give the image some life.
- as for smooth terrain, whatever you import from wm is straight from the wm data. also you can use the displacement tab on shaders to add some displacement into them and give it a little more bumpiness or whatever you want.
- have you given the wiki a look?
http://www.planetside.co.uk/wiki/index.php/Terragen_2_Tutorials. I've posted some video tutorials that goes through the basics of how to get going with all the major things in tg2.
