Someone already mentioned it, but MANU GINOBILI.
If Manu is healthy, the Pistons have no one who can contain his slashing ability, not even the long arms of Tayshaun Prince. So, in order to counter that, the Pistons coaching staf have to design defensive schemes to either limit Manu's touches from the top of the key where he can set his defender up for a one-on-one move, send different looks in order to trap and force Manu to pass the ball earlier than he wants to, or find wrinkles in the Spurs' offensive sets that will allow the Pistons to run different help defenders when Manu breaks down his defender.
While Tim Duncan is generally the source of success for the Spurs, and both Tony Parker and Chauncey Billups have had wonderful seasons, I really do think Manu v. Tayshaun is the match-up that will be the most decisive in determining the winning team, should the two teams meet in the finals. Tayshaun will not only have to try to contain Manu, but he will also have to be aggressive on offense, since Bruce will be covering either Rip or Chauncey, the two main offensive options for the Pistons.
Should the Spurs win, I suspect Manu Ginobili will be one of the more important determining factors.