In an interview over at GameSpy, Blizzard's lead content designer Ion Hazzikostas explains why Blizzard hasn't adopted the full open-tagging feature for World of Warcraft.
Open tagging, which has been popularized by high-profile MMOs such as RIFT and Guild Wars 2, refers to the ability to earn kill credit and experience simply for doing X% of damage to an enemy, regardless of whether or not you first tagged it. In World of Warcraft, the first player to tag an enemy gets full experience and credit, though Blizzard has made some exceptions recently for named quest targets and world bosses (tagged by your own faction).
"Part of the concern with open, free-for-all, open-tapping is that a lot of the game becomes more about tapping everything and tagging everything a little bit, especially when other players have already tapped them" he said. "Thirty seconds later, your quest completes because the other people killed them all. That's not an ideal way to feel like you're engaging in combat."
Find out more about World of Warcraft.