- Will there be more than 5 databases on a Mailbox server? If yes, you need Exchange 2010 Enterprise edition.
- Will you have a database availability group (DAG)? If yes, then you need Windows Server 2008 Enterprise edition for each Mailbox server in the DAG.
The only difference between Exchange 2010 Standard edition and Exchange 2010 Enterprise edition is the number of databases supported on each Mailbox server. There are no other differences. Database physical size is unlimited for both versions. DAGs are also supported on both versions.
A DAG requires the failover clustering feature that is only available in the Enterprise edition of Windows Server 2008. Failover clustering is configured and managed by the Exchange management tools, but it needs to be there.
It's worth noting from a design perspective that if you have a DAG and at least three copies of the database, then you can consider going backupless. In which case, you will likely use fewer large databases. Consequently, you often do not need the Enterprise version of Exchange Server building a DAG.
There is also an Enterprise Client Access License (CAL) for Exchange 2010. This CAL is required for all users accessing premium features. This includes unified messaging, the new personal archive feature, per mailbox journaling, and a number of other things. The Exchange Management Console identifies features that require an Enterprise CAL. A reference is here: