Working on AD I found that permissions play a huge part in allowing access to directory objects.
I think that creating a new abject without using any kind of elevation of privileges is a very bad
idea as you will get an exception that is not very useful. So to counter this heart ache we use elevation.

Elevation Code 

DirectoryServicesPermission dPermission = new DirectoryServicesPermission
(System.Security.Permissions.PermissionState.Unrestricted);

dPermission.Assert();

Hope this helps with AD!