The main difference between Exchange 2007 standard and enterprise is clustering. With enterprise you can do continuous cluster replication and single copy clusters. Both support local continuous replication and standby continuous cluster replication.
Now there is something else you need to be aware of as well. What is the difference between Exchange 2007 standard and enterprise client licensing. This is different than server licensing above.
The enterprise client license gives you access to unified messaging, advance compliance, Forefront security, Exchange hosted filtering, hosted antivirus, and hosted spam.