Combine Roles : A reduction in the number of available roles to two: a Mailbox Server and an Edge Transport (intended only in certain Office 365 hybrid scenarios).
Outlook on the web (formerly Outlook Web App) : UI changes
Office 365 hybrid : The Hybrid Configuration Wizard (HCW) that was included with Exchange 2013 is moving to become a cloud-based application. When one chooses to configure a hybrid deployment in Exchange 2016, you'll be prompted to download and install the wizard as a small app.
Messaging policy and compliance : New DLP and Archiving/Retention/eDiscovery features.
For more detail on new features, see the following Microsoft TechNet article: Whats new in Exchange 2016.aspx)