OK - If you think about all the greats, they all had something in common, and that is that they all somehow had their own voice, their own sound, their own ideas. Easier said than done, and it's extremely rare when one encounters it. Most very good pianists are still in some kind of hero-worship mode, modeling thier performances after Horowitz, Rubinstein, Richter, their own teacher, etc. and really they don't know a damn thing about the music that they are playing.
Oh, and add also technique, tone, accuracy, sense of structure, pedaling effects, sense of rubato, variey of touch, sensitivity to harmonic change, personal charisma...gee, am I forgeting anything? : )
Basically, it is not easy to become "great"...