A couple of thoughts here…
First, I would like to see a little clearer explination of Psychic Disciplines. I gather that, in Only War, they are purely descriptive and psykers are free to select their powers from any Discipline, but I'd like to see that more clearly spelled out. Perhaps just a short paragraph talked about the Disciplines, for the sake of the newbies.
Second, I'm disappointed that most of the Disciplines don't include "basic" powers. Telepaths can't read minds or speak telepathically. Telekines can't just movie an object and Pyrokines can't just start a fire. I know the combat powers are "sexy" but these basic powers would be useful for game play. Even Imperial Guard Psykers ought to be able to do more than just blow things up.