Actually, Kournikova herself packed quite a punch when she burst onto the scene, it's just that the lieks of Williams, Capriati and co have completely upped the levels required to compete in todays game.
Martina will be missed, but she is not quitting because of injury, rather because she can't handle the power-players. So she certainly is a quitter, but such a talented player, I'll miss the snobby little bitch.