I change my opinion with the colors  (forget the quick edit i made earlier, your colors really are brilliant  )

(actually i'm too creating a Wesnoth, Age Of Wonders kinda game, i'll maybe make a new topic after modeling the game couple painfull months )

I think i have to study this more before making any updates 

sorry for double posting but here's an update 



I updated the mountains and clouds as you can propably see 

i think the grass in fine and at a good height and that it's the background hills, being the same value as the character, that are what make him disappear.

value and color can be made a bit more important than they are in this piece.  for instance, here's a quick recolor:



This has a very different feel and actually may read better, despite being softer.  It may not be what you are going for but i'm just trying to demonstrate how color can set a mood rather than just define materials.

Thanks for the edit 

after ndchristie did that edit i started working on the image and here's what i've done so far:



and here's the same thing but played with the colors a bit 

(which one's beter ?)

Still don't actually what to do with grass and is the sky too dark in both images