Basically all (or some) of the information previously stored on Domino Directory via Person document form would be instead stored on an external directory (preferably LDAP). I am aware that there are informations that is exclusive to Domino like mail server and mail file. For these information if possible additional field should be created on the external directories to store them but this is technically not possible they can still be stored on Domino Directory.
Another critical function would be the ability for Domino to authenticate it's user against that external directory thus eliminating the need to synchronize your password over multiple directories. I don't really like password synchronization since they took time to "replicate". I am not saying that Notes should discard the usage of user ID. With ID Vault I think it is still possible for ID to be downloaded to local terminal whenever a user login and deleted if they log out or close the client.
I am well aware that what I suggested sounded very much like Directory Independence but said feature is yet to be published and thus is feasible to change in implementation or even being canceled all together.