1) The "Child of Prophecy" Routine - The people are expecting a hero who fits the PC's description to arise, and lead them. They won't follow the Mage or Cleric.
2) Prophetic Dreams - Anyone can have these, be they Fighter, Rogue, or Spellslinger. Only those who dream have the knowledge come to them.
3) "He Is The Key!" - Only someone with the "X" Special Ability can do Y. The Fighter is the only one who can do "X".
4) Secret Knowledge - Anyone can find a secret. If the Fighter discovers the scroll telling how to operate the ancient artifact, and it bonds to him, then only he can operate it, even if someone else later gains the same information.
5) "The Child Who Has Died Is The King Who Will Be!" - Similar to "Child of Prophecy", but less mystical... The PC is a descendant of a bloodline, and only someone of that lineage can open the gate/pass through the door/summon the dead to the stone of Erech, or whatever. (Be very careful with this one, though, as if your Fighter dies, the PCs then have to find someone else of the same bloodline!)
6) The Gift - As above, but the ability to do "Z" has been given to the PC by a godlet, etc.
7) PrC Ability - As "He Is The Key!", above, but it is a Prestige Class Ability which is important.
I'm sure there are many others...