What do you want? A title? An achievement? These days you want to work on your faction races. For Alliance: Glineas, Exodar, Stormwind, Ironforge, Gnomeregan, & Darnassus. For Horde: Bilgewater Cartel, Silvermoon City, Undercity, Orgimmar, Darkspear Trolls. The easiest way to do this is by questing & wearing a faction tabard when you do random dungeons. (Note: these will not work in Burning Crusade Dungeons, but will in Cataclysm, the redone Vanilla dungeons & Wrath of the Lich King dungeons.
Factions that were important in Vanilla, BC & Wrath are no longer that important these days. My main point would be level to 85 then concentrate on reputation with the Cataclysm factions as these will give you the new items you will want when you reach endgame content.