When shooting landscapes, you need to think about the direction of the light and how it's falling on your 'subject'. The first shot, for example, might look better if it was shot at a time of day when the sun is behind your position...hitting the front of the mountain and lighting up the forest. Although, if you want a dark feeling photo, then you are doing OK.
Same thing for the 2nd one. Those cliffs would probably look great with low angle sunlight...which would mean very early morning or late evening. Not only does the angle of the light add to the photo, but the color of the light is warmer at dawn & dusk.
The third one has some great potential with that winding stream/river...but, to me, it's too dark and/or has too much contrast. The trees are just too dark.
As for what makes a good landscape...that's pretty subjective. Personally, I think a good landscape draws the viewer into the image...as if they feel they could walk into the scene. It helps to have something to lead them into and though the image. It also helps to have a strong focal point...something that catches and holds the viewer's attention. I think your 2nd photos has this with the clouds but the 3rd one seems to be missing a strong attention grabbing subject.