1) Effective first serve (high percentage 65+)
2) Ability to remain at least neutral off the second serve
3) 80% trading ball (rally ball) of both sides
4) Movement efficient enough to protect contact point
5) Ability to take time away from opponent (i.e hit early mid-court ball)
*I reserve the right to edit this list as I see fit and change my mind/have my mind changed
I would add to this the ability to neutralize an opponents serve and a good second serve return points won. Also, the ability to have a good transition game from defence to offence as all good pro's posses this.