One major difference administrators need to plan for when deploying exchange 2010 is the changes to the client access server. In Exchange 2007 outlook clients talked MAPI directly to the exchange 2007 mailbox server roles. In Exchange 2010 the only device that talks to the mailbox servers is client access server. Exchange 2010 client access server proxies the MAPI requests on to the exchange mailbox server and applies smarts. This stops the mailbox server being spammed with multiple TCP connections from different sources like it was in exchange 2003 and 2007. This is also adds to how Microsoft have achieved the 70% reduction in disk I/O with the 2010 mailbox server, see:
In exchange 2007 the client access servers did not use much resources. In exchange 2010 ensure you take into account the client access servers will under a heavier load dealing with all the MAPI requests from end clients.
There is a new service on the Exchange 2010 client access server called the "Microsoft Exchange RPC Client Access Service" that is responsible for talking MAPI to the outlook clients on the internal network. The RPC Client Access service also talks to active directory on behalf of outlook clients, something that outlook use to do directly! Outlook connects to an NSPI endpoint on the Client Access Server, and NSPI then talks to the Active Directory via the Active Directory driver. The NSPI endpoint replaces the DSProxy component as we know from Exchange 2007.
This not only improves the consistency, when applying business logic to clients, but also provides a much better client experience during switch-over and fail-overs when you have deployed a highly available solution that makes use of the new Database Availability Group (DAG) HA feature.