You can find a list of changes in the MailChannels blog.
The most user-visible change is that port 587 is finally recommended for end-users, reserving 25 for MTA-to-MTA traffic. Also, use of SMTP for end-user clients is discouraged, pointing them to the Message Submission for Mail (RFC 4409) protocol.
For server operators, a lot of blessings to current behaviors:
- you can now legally reject an email after the DATA is complete: yes, most anti-virus/anti-spam appliances already did that, but it was not legal;
- timeouts are legal too.
It seems a pretty decent upgrade.