One reason the mode has eluded release is its lack of depth. "Simply fighting each other with no other objectives or choices to make gets old relatively quickly. We've brought a lot of people in to try out Team Deathmatch and, while some found it entertaining, most of our testers didn't feel like it was something they'd want to do beyond a few hours," Wilson wrote. The mode requires more varied objectives and lucrative rewards to compel players to keep playing over time, Diablo 3's lead designer added.
Another issue is class balance in a game designed with Player vs. Environment scenarios in mind. "In a casual PvP mode, something equivalent to a WoW Battleground, this would be fine, but Team Deathmatch felt very hardcore, and it put a laser focus on class balance in a way that we didn't think would be good for the game as a whole." Because of these issues, Blizzard has decided to go "back to the drawing board on a new replacement for Team Deathmatch." Whatever the mode ends up evolving into will indeed be free, Wilson noted.
Wilson adds Blizzard will add "Dueling" soon, due to fan demand. "First and foremost, if our original goal was to support dueling, then we're not achieving that goal very well if we don't actually give players a way to duel in-game," he wrote. Dueling will be added to Diablo 3 in patch 1.0.7., set to arrive after the new year.