Full control for player's camera, keystrokes, and mouse (as you state above, are needed.
after that sure speech would becomes as necessary/complementary too.
an sort of pointer addressing to player's coordinates, to knows each moment, the exact player position on the map. Non only respect to entity position.
And of course a full animate caracter being able to walk, run, jump, climb walls, swim, dive under water.
Able to express emotions, pain, fatigue, breathing hard by the lack of air, etc.
Cutscenes are often needed too.
Despite all this, it remains a good idea.
I'm thinking in a genre like Syberia, (I love Syberia), Or Tom Ryder too. (The range of motion of Tomb Raider, is so accurate).
3com