I'll just judge your first posted terrain (The jungle) since it's easiest to comment on and should cover the rest in a way.
What makes this terrain look plain is the fact that there are hardly any trees where there should be trees. For example, around the waterfall; which is in fact very narrow, in my opinion. The choice of trees you should place mainly in a jungle would be palm trees, which I see you have a few of in there already. Make more!
When terraining a jungle, you must take in account that jungles are very well vegetated. They're even more vegetated than most forests. So, be sure to place a lot of shrubs, ferns, bushes, rocks, whatever that looks jungle-like, get it in there while still maintaining a good jungle environment and atmosphere. Be wary of each object you place. Try to avoid making them clip through each other, but do not space them apart. This also means do not randomly place objects and try hard to avoid making the terrain look that way.
Fogs for jungles vary. Greens, grays, blacks, yellows, browns, and sometimes purples and blues go well with them. Most of the colors do actually. White would be one of the only colors you should stay away from when doing a jungle theme.
Jungles have a very peculiar terrain style. It's always varying and can be located any kind of geography as far as landscape goes, not weather/climate. Mountains, valleys, hills, and even plains you'll find jungles. Jungles are also located on the shores of islands as well.
It may be best for you to research the theme of your terrain you wish to make about its history and generally how they look. It seems to help terrainers design their landscapes more realistically and appealingly in the end when they are constantly taking in the guidelines they've learned over the time both terraining and getting feedback - such as mine - at the Hive and also their research.
Cluttered areas and empty areas are two things that a lot of beginner terrainers tend to get out of my advice. I don't want you to place objects too much all over the map close together, but not too far apart either. It keeps the terrain looking clean, fresh, and doesn't make small children want to gouge their own eyes out with wooden spoons found in Spain. The only time you might get away with empty terrains are in desert and tundra themes. As for cluttered terrains, maybe jungles and forests. However, you do not want to take that risk. I do not recommend clutter and emptiness, as it usually results in simple ugliness. Take care and pride in your work. Put a lot of effort into it and make sure you add lots of little details too. All good terrainers create terrains that are outside of the box ideas and are taken from usually artistic angles as well. As for you, whom is not exactly a very experienced terrainer (My way of politely saying you aren't good at terraining) you should not focus on such approaches in terraining until you are sure you are a good terrainer and that you can capture unique ideas through your terrain.
~Craka_J
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