Most modern engines can create terrain from heightmaps. A heightmap is a grayscale texture where black represents down and white represents up. It's quick and easy to have a mountain and valleys with it. However, a heightmap can only control the Z axis. It cannot handle more than two dimensions, meaning that we can't have tunnels and bridges unless we do it with 3D models. Hourencens is against the usage of heightmaps for cliffs, sharp edges and the attempt to add variation to the terrain by means of random noise filters. It just doesn't look natural at all.

Terrain texturing. The same discussion about contrast is made here by Hourences. The multiple terrain's textures have to be cohesive and have a good combination of colors and style. For example: to have mountains with brown terrain with random patches of green grass won't look good, both because of the color contrast and the random placement of the grass. One technique he gives is to use an intermediate texture to make the transition between two different textures smoother. For example: add some rocky texture between the grass and the road of dirt. In between rocks and the terrain there can be grass or smaller rocks with a different texture to make the transition smoother too.

In here I'd comment that technology is a limiting factor because some older tech won't be able to have multiple layers of textures and blend them with transitions. The solution for such cases is to pre render the texture's layers, merging them on one texture. In case there are severe memory constrains there isn't much room for large and/or multiple textures.

Rocks. In here Hourences makes an interesting statement about feelings, atmosphere and how that connects to the shape of rocks. If you think about harsh weather conditions strong winds and storms for example. The rocks are probably going to have more rounded shapes and more trails where the water flows down. If this is an MMORPG this ties in with the types of enemies the player is going to encounter there. Smoother would equate to easier. Now think about aggression. Spiky mountains would feel more aggressive. The enemies in the area would be more aggressive to match that atmosphere. If I stretch this concept of shapes and atmosphere a bit I can relate it to the graphs that Hourences made to explain contrast. Smoother graphs vs Rougher graphs would equate to friendliness vs unfriendliness.

About natural looking rocks the first thing that I'd look for is for pictures of real world landscapes. I'd pay attention to weathering processes and gravity. Rock formations where there is a lot of fractures, cracking, landslides and other erosion processes are good places to start from.

Horizon. This is pretty simple. Avoid cutting off the world's edges like a sharp knife cutting meat. Mountains that have nothing behind but void should be avoided. In here I'd like to comment that this is also a technological limitation. Sometimes the hardware cannot handle rendering open vistas and concessions have to be made.

Sky. Again, technological limitations mean that the game won't be able to produce any type of sky. The hardest limitation is that it's not easy to render realtime dynamic weather. Static weather on the other hand is easier because we can rely on static paintings. The most obvious mistakes here regard color and time of day. If there is a thick cloud cover the ambient lighting can't be very bright. If the sun is shinning from the South, the shadows on the ground have to match that. Else it's going to look fake and wrong. The sky must have clouds, moon, stars to convey emotions too. If it contains nothing but a single color it becomes boring, unless it was intended.

Textures. The most glaring mistake is to have misaligned and/or wrong scaling. Another mistake is to mismatch textures, such as clean floor with dirty walls, or different styles that shouldn't be used together. In here I'd add that sometimes the storyline can have excuses for that. We do have some degree of freedom, but at the same time freedom doesn't mean arbitrary choices. One way, maybe the strongest way, to keep textures under a common theme is to use a limited color palette. It doesn't mean that all textures from a certain group should share the same colors, but that they obey to some style. An example of this is Star Wars. The textures regarding the empire always have a predominance of white, black and grey.

Hourences tells that one often overlooked property of textures is brightness and saturation. When an artist makes a texture it's displayed on screen without interference from lights. However, in game, there is lighting and the light has color and brightness. Textures have to account for different lighting conditions. Programmable post processing can do something about textures and light, but better solve those issues at the origin than trying to rely on post processing to fix it.

Materials. With the ever advancing technology textures are closer to real life materials. We can control properties such as how much light it reflects, how much light it absorbs, how much smoother or rougher it is. This means that more power brings more responsibilities and more complexity. Reflectance, transmittance and other properties dictate the environment's mood because materials convey coldness, warmth, receptiveness, neglection, etc.