Most game designers treat fighting as the fundamental activity in an MMO with the result that combat systems are typically both rich and varied. Different classes can have markedly different combat styles. Some classes bash in the enemy's head and get drenched in blood, some run the enemy around in circles whittling them down as they go, while still others nuke at a distance. Combat can also differ radically depending on whether you're playing solo or in a group. In addition, there's the immense difference between fighting a human opponent in PvP and a computer-controlled opponent in PvE. In order to keep the discussion manageable within the confines of a column like this, I'll limit consideration to solo PvE melee combat in fantasy-based MMOs.
The basic difference between auto and active attack takes place at the level of game mechanics. In an auto attack system your character will continue to attack the opponent once you instigate combat. Depending on the number and strength of the opponents, you may survive the encounter after having done nothing other than start the fighting. In an active attack system the player must instigate each individual attack. If you click on the opponent once and then don't do anything else you will either win with a one-shot kill or get your ass hammered into the ground.
If that were all there was to it, active attack would have the clear advantage over auto attack. Doing something is always more interesting than pressing a key and then watching the game play itself. But that's not all there is to it. Auto-attack systems have become very sophisticated by giving the melee character an arsenal of spells and special attacks that give the player plenty to do during combat and move the discussion from the realm of game mechanics to tactics.
If there is an important difference between active and auto attack, it lies at the level of single-unit tactics. In most auto attack systems, tactics comes down to managing both a resource such as stamina, energy or mana that powers special attacks and the special attacks' cool down timers. An additional layer of complexity is added in some systems by buffs or debuffs that set up a subsequent attack. Players will typically settle on a series of attacks that produce high amounts of damage and allow the player to launch a steady stream of new attacks as cool downs end and the attack resource replenishes. The result is that the player is almost constantly engaged by pressing keys during melee combat.
Age of Conan's active attack system is built on a directional attack mechanism. Attacks can be made from the right, left or center with each direction being controlled by a different keypress (the default keys are "1", "2", and "3" for left, center, right, respectively). Special attacks in AoC are called combos. The player launches a combo with a keypress after which a sequence of directional attack icons appears on the screen. If the directional attack keys are pressed in the correct sequence, the combo is executed. Combos have a resource cost and cool down timers and, as is the case with the auto attack systems, tactics mainly involve managing the resource and the timers so that a steady stream of special attacks can be launched.
If you look at this in terms of the player's engagement with the combat system, auto attack and AoC's active attack are very similar at the level of individual unit tactics. In both systems the player is more or less constantly engaged during combat as they execute a sequence of keypresses in a specific order. Players will typically find a series of special attacks that best balance resource management and cool down timers and execute that series at the beginning of combat much of the time. There are always exceptions but the first three to ten or so keypresses are often the same whenever you engage in combat with a mob. You execute your favored string of special attacks and then decide what to do next based on the outcome. The key sequence differs from game to game but whether you're using AoC's active attack or a different game's auto attack you almost always begin combat with an unchanging sequence of keypresses that becomes more or less automatic over time. If you stop launching attacks, different things happen in the two systems. In active attack you die, in auto attack you may win the fight as your character continues to bash away at the enemy. However as long as the player remains an active participant in the combat, there isn't much difference between the two approaches. At least insofar as active attack is currently implemented in Age of Conan.
It doesn't have to be this way. Combat in both auto attack and AoC's active attack is less engaging for the player than it could be because typically you don't have to react in any way to what the opponent does. You run through the same series of special attacks all the time because it's fairly successful no matter what kind of enemy you're facing or what kind of attack it's using. You can usually ignore what the enemy is up to while you cut it down.
The solution to this problem is a system that asks the player to respond to changing combat circumstances as they occur. Surprisingly, Age of Conan takes a welcome step in this direction with their use of shielding and then abandons it. In AoC computer controlled enemies have three shields that can be deployed in different configurations. The shields can be spread evenly against left, center, and right attacks, two shields can be deployed against attacks from one direction leaving another direction with one shield and the third direction undefended, or all three can form a bulwark against attacks from a single direction leaving the other two directions vulnerable. Basic weapon damage (called white damage) is inversely related to shielding. Attacking against three shields does very little damage; attacking against no shields does a lot. The brilliant thing about the way AoC implemented this system is that the enemy shifts its defenses in response to your attacks. This means that the sequence of attack keys the player presses doesn't become automated. In order to attack effectively the player must react to developments that take place during combat by changing the direction of attack as the enemy changes its shield configuration.
In terms of player engagement, active attack plus changing shields is a great idea. Unfortunately, Age of Conan does not carry through with it for combos. The sequence of keys you press to execute a combo is unrelated to the opponent's shielding and combo damage is usually much greater than white damage. This means that as your character develops and you rely more and more on combo damage, you quickly get to the point where you ignore shielding completely as you launch combo after combo at your enemy. The result is that combat is much more engaging for the player at early levels when you are relying on white damage and are reacting to changing shield configurations than it is at later levels when shielding becomes largely irrelevant.
There are several ways in which Age of Conan could have enriched their active attack system by keeping players focused on reacting to changing circumstances during combat. Combos have levels. When a combo goes up a level the pattern of directional keys you press to launch the combo changes. However, at any given level the direction keys that launch the combo do not change. It's always the same sequence. A simple method of eliminating the problem of an automated attack sequence would be to randomize the direction keys every time a combo is launched. The player would be more actively engaged with combat because he wouldn't know which keys he needed to press to complete the combo until after the combo was launched. A system like this could be made even more engaging by putting timers on the combo's direction keys. As the combo increases in level, the player is given less time to press the key. If the player responds within the time limit for a level X combo, they do level X damage. If they're too slow, they do level X-1 or X-2 damage.
Another way to make Age of Conan's active attack more engaging would be to allow combos to be affected by shielding. Add a keypress controlling the direction of attack to the sequence needed to execute a combo. The combo sequence would then be (a) launch combo, (b) direction of attack, (c) . . . (k) (randomized) combo keys. Combo damage is then scaled like white damage by the enemy's shields. In a system like this the same key always launches the combo but all the other keypresses depend on the current situation (assuming the combo keys indicated by (c) . . . (k) are randomized).
At the simple level of game mechanics active attack is more interesting than auto attack because it demands that the player respond or die. Players don't usually play at the level of game mechanics, however, and in terms of player engagement at the tactical level there's very little difference between active attack as currently implemented in Age of Conan and auto attack. But it's early days yet. Active attack has the potential to become much more interesting if attack systems are developed that keep the player responding to conditions that change throughout combat.