3D quality imo is a matter of choosing a very detailed texture. The mesh could look like a brick and all you really need is a quality texture to make it look good. Then in your final render lighting and shadows are key to making the 3D look real.
I start most of my projects with a low poly generic mesh. Then subdivide the mesh only in places where I need more details. For example the face needs more detail than a head (which will be covered with hair), eyes and mouth are higher than the nose. Subdividing the body parts is a waste of time in most cases. If you are making a movie then use low poly models for distant shots and the high poly for the tight shots.
I also keep a good contact list of people who do textures (both for free and profit) or can find them for me. The best texture wrangler I found by far is Jabhacksoul. He works in Blender and uses the Gimp to make custom textures. If he has the texture "lying around" or doesn't have to make it for you, then he will most likely give it to you. I got a load of great custom textures off him for about $10. Google his name he isn't hard to find. There are sites that have textures as well like free3dstextures.com
After you UV your mesh with a good texture, then the lights are real important. Use at least 3 light sources and one to cast shadows. Place a soft yellow local light behind your subject. To the left of the camera place a soft blue local light about half the height of your subject. To the right and above the camera place a spot of white light directed on the subject. Set the spot to raycast a shadow and you will have a better quality 3D rendering.
