There is a lot of confusion about the JET database engine used in Exchange. This is fairly similar to what Notes has done. The Notes directory is called the Public Address Book, and is implemented on the same DB engine as the rest of Notes. They also rev the DB engine every major release to add function, performance, scale, etc. This is handled by Exchange.
Notes also handles access control itself.
ACLs are placed at the database and also the individual document level. Notes ACLs are provided at the server, database, and individual document level. Notes replication is sort of built into Notes. There is a separate process to handle replication, that runs on the Notes APIs, but there are lots of APIs tuned specifically to support replication functionality. There are tradeoffs.