RPG Combat: Tanks, Threats and Aggro, Oh My

For his latest WRPG-focused editorial, Rowan Kaiser focuses about the recent changes in combat mechanics in RPGs, and specifically the concepts that have been introduced in MMORPG, such as threat, aggro, etc. Here's a snip:
Multiplayer games offered a conceptual shift as well. In a single-player game, even one with a party, an enemy attacking one of your characters was still attacking a projection of "you" within the game. The challenge is dealing with that. Online, that's different. Your character is one of many in a grouping, and if you're not being attacked, you have no challenge. Moreover, almost every online RPG has been real-time, and would suffer from the same issues as single-player real-time games in terms of chaotic enemy attacks.

So new systems were devised, although they were rarely transparent to players. In Ultima Online, the character who seemed to be doing the most damage got "aggro," or targeted by enemies. Given its free-form skill system, where you could be a powerful mage and wear the best armor, this wasn't a huge issue. By the advent of Everquest, however, with its much more specific and rigid class system, concepts of tanking solidified (it's possible that other games used similar forms to Everquest, but EQ certainly popularized them) . It makes sense: classes have to balance amount of damage and survivability. Heavily-armored warriors who can also deal massive damage will have an inherent superiority to mages or rogues. So tanking warriors do less damages. But then having enemies attack whoever's doing the most damage, or randomly, the tanks never have a chance. This requires a mechanic to control aggro: threat.

"Threat" is built up by actions outside of simply damaging an enemy. Healing generates threat, attacking generates threat, and most importantly, tank classes have skills that generate threat. "Taunt" skills that force an enemy to attack a tank are the most common, and even existed outside multiplayer games. By the time of Everquest, you had different ways to manipulate threat: "Pacify" skills for healers to lower threat, and various other skills for warriors to generate it without doing damage: for example, "Sunder" was the preferred choice early in World Of Warcraft. A character no longer simply was a tank by virtue of hit points and armor, but had to engage in the act of "tanking," or using the proper skills to generate threat and maintain aggro.

As with so many things, what Everquest began, World Of Warcraft solidified. Early boss fights and raids were often straightforward "tank-and-spanks," where one character held aggro while being healed, and others attacked the boss freely (doing as much damage per second as they could, or "DPSing"). But clever modders soon began to test ways to measure the previously opaque threat mechanic. Add-ons like KTM (the KLH Threat Meter) made it numerically clear. Now players didn't have to guess whether they'd draw aggro, but instead just had to watch a meter. I remember reading interviews with developers who said that the threat mods forced them to completely rethink their boss fights: threat's importance was even surprising game makes.