In Craig's article asking for feedback on SWG's Health-Action-Mind system, something interesting came up in the posts of Van Hemlock and Jonathon Stevens.
Hemlock said the HAM system bothered him in that it seemed like different team members were competing against each other during combat. One person would be using a weapon that damaged the enemy's Health, while another would be using a weapon that damaged Action or Mind. If any one of the three pools was drained completely, then the enemy would die. So if the first player was able to drain the enemy's Health before the other drained Mind, it felt like all that work against Mind was unnecessary and wasted.
Then Stevens pointed out that different abilities are linked to each of the pools. Each special attack drains one's own energy. To stun, you might have to spend Action points. To root, you might need to spend Mind points. So as you drain one of an enemy's pools, you also cripple his ability to perform certain actions. Different enemies favor different pools with their skills, so draining Mind might be most effective against one enemy while draining Health is more effective against another enemy.
Whether or not that SWG system is a good one, it highlights an interesting hurdle in most combat systems and game systems in general: a player must be aware of his or her success to take any pride and enjoyment from it. As in Van Hemlock's case, if the player does not notice the effect of his actions, then he will lose interest in performing those actions.
In psychology, this is known as extinction. The important nuance to note is that a person will cease to act if he or she believes those actions have no result, regardless of whether or not there actually is a result.
So, a game must not only give the player agency (the ability to affect events), but also ensure that the player is aware of that agency.
In the scenario Van Hemlock described, the game relied on player knowledge. He could see the Health/Action/Mind pool being drained. But to translate that feedback into an awareness of his being effective in combat, he had to know that (1) skills are dependent on a particular energy pool, and (2) which pool this particular enemy needs for which skills. The combat experience was unsatisfying because it failed to provide the latter information.
But the enemy's HAM bar would have shown the depletion of a particular energy pool as the enemy used its skills, right? If it was using a Mind skill, the player could see the Mind pool drained.
Ahh, "could"! There's the problem. The game was providing the necessary feedback, but the feedback was drowned out in everything else that was going on. Just because you put it on the screen doesn't mean the player is seeing it. If there's too much information (the player's HAM, each enemy's HAM, each ally's HAM, terrain/environment, the enemy's strategy, the player's strategy, etc.) and it's too spread out (seven HAM displays up top or in the corner, the enemies moving all around the screen, barriers and other relevant environmental objects up close and in the distance, nearby NPCs who will join the fight if you move too close, etc.), then some of that information will probably be missed.
In this case, the problem was largely with the user interface. But a UI isn't the only feedback system which must this sort of problem into account.
The other part of the problem was that many players don't want to merely maim or counteract an enemy; they want to be more directly involved in the enemy's death.