Disadvantages of active directory in comparison to other directory services?

1. Active Directory is a Flat File Database, and as such does not scale well to distributed locations. On very slow links site replication can take upwards of 4hrs, or fail to complete before next replication which will cause errors.

2. Active Directory fails full LDAP compliance, which limits what open source or 3rd party products can be deployed (as such, costs increase exponentially)

3. Active Directory is OS dependent and cannot be installed on any platform other than Windows server. (This increases licensing costs and reduces reliability).