Not everything in virtual worlds are indoors. Sometimes you have to go out in the sun.
Plants and trees are often low polygone stuff that can fill up between the buildings or complex objects. Its important to place some open spaces in your simulation. We are not always indoors in real life. Well maybe i am. Have to buy a laptop i can take out on the grass.
But anyway making open spaces also saves polygons and textures for the more confined spaces of a game or simulator and makes it more exiting when you go from inside to outside.
Another good thing is that there are a lot of people that create plants for free ready to download. So it also saves you some money.
Another very nice way to create a outdoor landscape is to use water. Water makes a moving and often sounding addition to the open space and puts in a little action if your plants are static.
Sound: To ad reality.
Every world needs a full range of sounds both in the background and for vehicles and gadgets. Most sounds can be created by just recording it and then do a clean up on the file. Others requires you to create the sound from scratch by mixing midi and wave sounds. A sound can take as much time to make as total 3D build and its often really important for the ambiance of the scene.
I have been doing 3d graphics for a long time now. Its all about that little triangle. How many and how to use a texture or not. Just to make it take as little process power as possible. Effective use of spaces to separate groups of objects and now days how to make download as small as possible. Then there are some more things to think about like shadows, ambient occlusion and what kind of shaders that the viewer software uses. Reusing objects and textures are important. Trying to cut frame rates on the graphics card and making the build a nice experience for the viewer.
I see some great builds in games and in Second life that sometimes blow my mind. How did they do that? When I analyze it. It´s always really good use of triangles, space and texture. That is what I want to do. That is what 3D graphics is all about.
A world do not only contain landscapes and buildings. Every world or game are full of usable objects that the player can drive or use in different ways.