For those who don't play Priests, Shadowfiend is a level 66 spell for Priests that summons a short duration pet to attack a mob. The damage the pet does comes back to the Priest in the form of mana. It's Blizzard's way of giving Priests some form of mana regen in a dps form.
Player Xanthin of the Gnomeregan server noticed something odd about his Shadowfiend pet after the latest patch. In a fight, the Shadowfiend returns about 350-450 mana from the pet's crits. But after the patch, his pet returned much more mana to him. In the neighborhood of 800 mana and as high as 1260. What had changed?
CM Nethaera stepped in to enlighten confused priests: Shadowfiend scales on +damage gear. Since all +healing gear got an extra boost of +dmg, the Priests' pet is returning much bigger numbers than before. This Burning Crusade spell that was only used by some Priests may have just been upgraded considerably in popularity.