Three main topics for this.

Earlier this week the Sith Warrior / Jedi Knight Developer notes for the expansion came out on SWTOR. Check out the Developer Blogs for Guardian/Juggernaut and for Sentinel/Marauder.

The last bit is short, but ultimately important if you're leveling a character to 55 as fast as you can to see the Shadow of Revan expansion when it releases. While Shadow of Revan has a non-class specific primary storyline, there is a secondary storyline which is tied into each of the classes individually. To receive the secondary storyline quest you must have completed Chapter 3 of your class quests. There is no requirement to have completed anything on Illum or Makeb.