When using mip-mapping, blocks like dead bush and tall grass have a dark outline around them, making them visually unattractive at a distance. This also has a minor effect on cactus.

The anti-alias used in the image preview nullifies the effect slightly, you may have to try for yourself.

This may be cause of mip-mapping however, it looks very bad mip-mapping is supposed to smooth textures out at low angles and long distances. Mip-mapping does not do this to other 3D games I play.