QGIS and Autodesk Map 3D allow users to style features using map units at a fixed, real scale (in metres/feet). this is very useful when a dataset is a point or polyline feature class, but has a physical dimension (road width, tower / zind turbine footprint etc). This would make it easy to display those features in maps in a realistic way without having to buffer the features and create a derived feature class.
It is partially possible at the minute using the "proportional symbols" option, but this is quite restrictive (only one style for all the features, and the size has to come from the attribute table (where either the circle radius or area has to be precalculated). In addition, the legend for layer styled using proportionnal symbols is not customizable. it just shows the selected unit (metres) which is not hepful.