There are a host of new things in Mysteries of the Sith . The latter turns your clumsy and random blaster into a long range sniper tool. You'll also be able to find seeking rail charges which home on a target. Force powers work differently this time around. Rather than Dark and Light side your power simply drains when you do evil things.
Some powers are also mutually exclusive in the game as before.
On the new enemy side there are Hutts, torture droids, assassin droids, Scout Troopers, Noghri, and more. The new additions definitely added a lot to the game, and made the feel much different than the original. There are several changes in the multiplayer area. The first is a new multiplayer mode called Kill the Fool with the Ysamari. Everyone else will be out to kill you to take the Ysalamari of course.