Middle C
When starting to learn the piano, most books make their easiest pieces start on Middle C.  This is because the right hand can then play tunes using 5 notes (C to G) using white notes only (no sharps or flats).  It is also easy to recognize Middle C in notation.  It is also a help to find the correct place to sit at the piano (Middle C is opposite the player's "middle" (tummy)).  One problem can be that young children learning the piano may think that all pieces start on Middle C.  Pieces can start anywhere on the keyboard.