Exchange 2013 has introduced some new features that bring great options for geographical disaster recovery and High availability.
1. You no longer need a layer 7 load balancing. you can now use TCP based layer 4 transport load balancers. These are typically cheaper, and is a clear reason the TMG is being eliminated
2. URL redirection is now built into Exchange 2013, as it is now a true HTTP proxy at the CAS. It also serves as a stateless proxy for SMTP, which allows for pass through connections through to the mailbox server.
* Exchange will allow a client to cnnect to a CAS in one region, using a single namespace, then identify the URL of the CAS using URL redirection defined in Exchange 2013. Once identified, it will redirect the client connecting to the CAS in the same AD site (assuming it's internet facing) as the mailbox server that users mailbox resides on.
3. Geographical load-balancing using DNS - Round Robin DNS is a way for you to allow external connections to cache multiple DNS entries, and if one is not available, the other will be pushed in 20 seconds. What Microsoft also supports for single namespace HA is Geo Loadbalanced DNS, which will identify the DNS entry best suited for a client based on where they are connecting from (IP identification)
4. If you deploy a CAS with a DAG stetched across two sites - it will always route the SMTP connection to the DAG that your active mailbox sits on. However, if the DAG member you are connecting to is not available, it will create multiple SMTP connections and write to both DAG members with a copy of the mailbox. This feature is call Safety Net.
5. Managed Availability - All components of Exchange in terms of protocols on the mailbox servers has a recovery mechanism that is sequenced. Many of the actions taken are automated, allowing for built in recovery mechanisms.
6. Maintenace Mode - you now have the ability to put a DAG member mailbox serveer into maintenace mode to allow for it to be temporarily be in service and not be active in the Exchange 2013 installation.