Isnt it obvious? The same stuff people everyone else needs to min/max your gear set? To enchant gear, reforging, gemming, glyph changing, respeccing and profession leveling for the stat perks? Since PvP'ers need the best possibly stuff to remain competetive, one needs to spend several thousand of golds for meta gems, belt buckles, expensive weapon and bracer enchants, epic gems (in case of DS) etc.
One could argue that unless you are killing everything pre-nerf in raids you don't need stuff like epic gems or sha crystal enchants, however PvP'ers will need to optimize their gear to not be at a disadvantage in competetive PvP.