hmm =/. In my opinion I think the terrains are all a bit random. The first one, the trees don't really have a.....unified placement I guess? And ya I know, but trees aren't planted to grow like that, well they just seem out of place to me. That and its the same tree over and over again, same size. You could use some variation there.
The terrain itself looks alright, but the doodads just don't seem to have that great of a placement. I don't think I've ever seen a cactus growing out of a hill like that, they tend to stay on flatter ground, not steeper inclines.
A tip for the water and the ship, use deep water, not shallow, because seeing the ground underneath the ship, and the very faint water, makes it look really unrealistic.
The others have mainly the same problem, cluttered and random.
I'd advice you to check out some of the really good terrains on here, just to see how exactly the doodads are placed, the terrain is laid out, and the small little things that they do to add atmosphere to it. I'm not trying to discourage you from doing terrains, I'm just trying to let you know what it is needs work, otherwise you'll never get out of doing it, and you'll start to feel that it looks fine.
Good luck with future terrains.