Shameful because ripping models and using ripped models are considered illegal and unethical. However, in these days WoW models and other Blizzard models are already over ripped and used by extremely many people.
I honestly dont see any harm at using them, especially for terraining. >.>
But now, onto the terrains. For the first one, I really think it looks very monotonous, I'd recommend you spice it up a little, add more variation to the plant life, probably with a neat and nifty looking center of focus, like some great ruin up front or something, that and you could also fool around with environmental effects, such as rain.
For the second one. I, as opposed to 2-P, really like the fact that the roundness of the cloudsphere is visible, it gives the terrain a rather surreal feeling that I like. Though I'd recommend you fatten up the cloudsphere a little, so that it doesn't seem so stretched in the hight, and move the sun over to be at the center of the top of the cloudsphere. That and work on the lighting on a whole as well, right now it looks far too intense and unrealistic.
The third one I see very much potential in, though I'd have to say the execution of this terrain is rather poorly done. You've zoomed in on doodads with poor textures, making those poor and stretched textures very visible, and very bad looking. I'd recommend you have a look at the wooden doodads in the Refuge in Ruin map by _oGre_ and use them to re-create that mast, then finding some other doodads with better textures to replace the... Decor. As a final say I would also reccomend you move the sun over to be just over the mast, half-covering the mast in sunlight rather than having it behind a cloud which just looks odd.
The fourth one... Well, Ha.
- If I were to suggest anything, I would suggest you move the camera up and clip away some of the foreground, make the foreground entirely black and remove the lazer effect comming from the raindeers, and then work on some cool looking background for the rest of the terrain, possibly with some sun effect and mountains or something.