A recurring theme when placing decoupling caps is exactly how close to the power and ground pins they can be placed. This is in turns depends on the actual package dimension and mechanical space needed. It would be absolutely awesome if a mechanical package outline layer was added; in it we could associate outlines with footprints on a separate layer. It could even optionally be auto-generated from the 3D model if one is available, through a 2D projection and edge detector. A 3D preview helps, but often 3D models aren’t available and not worth the trouble to create. Also, a mechanical package outline layer could be part of the DRC.
The new version of the Kicad Library Convention (KLC) requires the component outline to be shown on the Fab layer (item 7.4).
In addition to what @hackscribble already wrote:
There is also the courtyard layer. The intention is that the courtyard layer is designed such that it enforces the space needed for a pick and place robot.
Or if you assemble your boards yourself the space needed for the solder iron. But in this case it might be necessary to enlarge the courtyard for some footprints. The only problem is that there is not yet any automatic check for courtyard overlap violations.
Also note that any official library is a compromise. It is designed with “one size fits most” mentality. (Some would say: “one size fits nobody”.) In other words: Check if the footprints you use fulfill your requirements. And if necessary create your own “approved” library that fits your needs. (The official library might be a good starting point.)
If you find footprints that do not follow the new klc it would be nice if you could not only change them for your use but share them with all of us by creating a pull request on the official library. (This only applies if the klc fits your needs or if you are prepared to do extra work.)