Editing RPG System Symmetries (section)

===Body Plans===
Symmetry with respect to body plans would imply that as far as the system is concerned a limb is a limb, and there are unified rules for dealing with characters with arbitrary body plans. This also means that while a humanoid body plan should also work, it shouldn't be the default with everything else being exceptions.

D&D pretty much ignores body plans except for granting extra attacks and a few other occasional details. GURPS tries to deal with this, but it's done as a series of exceptions to the humanoid standard. (Not to say that there's anything necessarily wrong with either approach. That's why this is under the unusual symmetries.)