I had a post drafted on adding roles and features, to begin a series of posts on server role deployment. However, Michael Neihaus has been kind enough to write up a great article on his blog. You can read it here: MDT 2012 Update 1: Roles and Features improvements
In addition, roles and features can be automated in the customsettings.ini. For a client use OSFeatures and for servers use OSRoles and/or OSRoleServices or OptionalOSRoles(Depricated, I think). These features use the properties in a comma seperated list. Example:
To install both ADDS and DHCP roles on a server.
Add the Install Roles and Features step to your task sequence.
In your customsettings.ini add this line:
OSRoles = AD-Domain-Services,DHCP
Remember this only adds the roles, not configure. That comes later.