Tips:
1. flatten the landscape, then raise it up in a hill.
2. For "cliffs" use the plateau tool and flatten the bottom inward, then "paint" your rock texture along the side of the hill - using something like clumpy grass at the top (over hanging) and grassy dirt at the bottom.
3. Use several kinds of trees from various sets: Ex: from Ashenvale use both types of trees, from the city use the summer tree, from Northrend use the dead tree.
4. Use rocks or rock chunks and enlarge them. For Rock chunks convert the category to "ramp" or Pathing blocker so it can not be attacked in game. Place a few large ones in your rocky cliff area, then use CTRL page up or page down to control the height. Creating a pile of rubble around the base of your cliff to act as a visable reason for pathing blockers You can go back and fill in with pathing blockers (if you desire) or you can change the pathing of the rocks. Rock chunks will give you the near vertical cliff face look.
5. Add environmental stuff (flowers, shrubs, bushes) and units (animals/critters: deer, raccoon, etc)
If you use rock chunks as a cliff face you need to balance our the stretch of the rock side against the needs of your game and what can be seen In the game. Usually 3-4 is about as big as you want to take it unless you are insetting smaller rocks inside or on the side to manage the stretched out look of giant doodads.
I threw something together to show you what I mean.