Speed is a must. Look at all the playoff teams and you'll see trend of great, speedy duos at the top:
Yankees: Soriano, Jeter
Twins: Stewart, Rivas
Oakland: Ellis, Hattenburg
Boston: Damon, Walker
Braves: Furcal, Giles
Cubs: Lofton, Grudzielanek (maybe not so much right there)
Giants: Durham, Grissom
Marlins: Pierre, Castillo