Email is fairly limited on the platform for the moment, featuring only the standard functions required to synchronize with multiple protocols, reply and forward messages and attach files. This is set to change with Mango with the introduction of threaded conversations and a combined mailbox.
Linking multiple accounts will enable users to separate addresses (such as work and personal) and pin a tile to your home screen for easy accessibility. Threaded viewing (and grouping) of your emails will provide a less cluttered interface, especially in work related mailboxes with multiple conversations taking place with numerous contacts. A huge improvement for enterprises will be the security improvements being made in the upcoming update with the support for more complex passwords.
We will also be seeing the WP7 team bringing threaded conversations to SMS.