While the design elements mentioned are critical, I think they are only part of the AD design. After AD is up and running, you need to design your OU structure. I prefer an OU structure based on an administration model keeping in mind the need to either locate objects based on geography or company organizational structure. Either way I then prefer to separate the objects further into sub-OUs based on object Type (ie Users, workstations, Servers, Groups). This makes it easy for delegation of rights as well as for applying Group Policy.
I think this applies to large organizations as well as smaller environments.