Keeper of Traken is a lot of things. There is a theater type quality to it with the costume and sets. This is the last serial for a while where there are no Earthling companions. Of course, this episode had no Romana.
The return of the Master is the most notable highlight in my opinion. Out the Ashes of the Deadly Assassins, and I do mean Ashes, the Master returns at his most desperate, grasping on to any chance of an extended lifespan.
We also have the introduction of a new companion, Nyssa. She's as delightful as ever, which makes the conclusion of this serial rather sad, for the Doctor's foe wears the body of Nyssa's father. Up until this point, this maybe the worst thing the Master has every done on Camera.