The gist of it is that when two creatures fight, instead of rolling for each to hit, only one roll is made, and there's a winner and a loser. Regardless of the results of the roll, somebody's gonna get hurt. The winner deals damage, and the loser takes damage.
Why this appeals to me:
- Obviously, no wasted rounds. No more double-whiffs when both sides fail their attack rolls. Every round counts.
- Instead of two fighters hacking at each other for a round, one of them gets a decisive upper hand during that round, and the other is put on the defensive, struggling just to survive. Next round, fortunes might reverse. This feels more natural to me, and more evocative, imparting a sense of ebb and flow rather than the toe-to-toe pitched battle depicted in standard combat.
- The winner-take-all results each round are dramatic and easy to narrate in a colorful and exciting way. One fighter capitalizes on the other's mistake to claim the advantage. A fighter battling a giant either dances away from the monster's attack and slips inside its defenses to stab at it, or is batted aside by the giant's mighty club.
- It feels clearer that an attack roll does not represent a single swing, but an entire round of maneuvers, thrusts, parries, wary circling, etc.
- A fighter can hold his own in combat against a foe he can't hurt. Say he's wielding an ordinary sword, and in melee against a wight. Under standard rules, he makes his completely ineffectual attack and then waits for the wight to attack and hopes it doesn't hit him. With this rule, he makes the single combat roll between the two of them, and if he wins, he holds the monster off. That sounds a lot more interesting to me.
- It merges offense and defense as a single function of a character's class and level, instead of the dichotomy of level-based offense against static defense.
If a character or creature is meleed by more opponents than it has attacks, it chooses which to apply its attacks. All others attack it with a -2 penalty to its CR - they are not threatened by its attacks, and so can attack more aggressively with relative impunity. If they win, they inflict damage, but the defender inflicts no damage if it wins. (Optional: the defender inflicts damage if its defense roll is a natural 20.)
Characters or creatures with multiple attacks can either concentrate them on a single opponent or spread them among several. Attacks spread among several opponents are resolved normally. If focused on one opponent, only one roll is made, as usual, and if it succeeds by a certain margin, the additional attacks succeed and score damage. Say, if the adjusted roll is 16 or higher, a second damage die is rolled. On a total score of 20, a third die, and at 23 a fourth. The Damage Reduction from armor is applied to each damage roll. (Reversed for monsters, so two attacks succeed on a roll of 5 or less, three on a 1, and four on an adjusted total of -2 or lower.) Thus, an inferior opponent is less likely to deal extra damage to a superior one, and a superior one is more likely to deal extra damage to its inferior. Note that if the attacks are not concentrated on one opponent, no single opponent may be subjected to more than one. If a creature with three attacks fights two opponents, it can either focus all three attacks on one of them, or use one on each and lose the third.
Monsters would do base damage based on the sum of all their attacks divided by the number of attacks. For instance, a monster with three attacks capable of a maximum of 24 points per round would do damage of 1d8 or 2d4 per successful attack, even if in the rules-as-written it does a claw/claw/bite routine for 1d6/1d6/2d6. Monsters' Damage Reduction would need to be deduced from the monster's AC and description; some might well have DR greater than 3.
Tactical options that work particularly well with this system include:
Set spear: Interposing a spear or other weapon with reach gains a +4 bonus to CR against a single opponent, if that opponent has not yet closed to melee. The bonus applies until the opponent manages to make a successful attack, or until the spear-bearer lets his guard down. A character with multiple attacks may maintain this guard against multiple opponents.
Reckless attack: The combatant using this tactic hurls itself at its opponent heedless of the other's attacks. If it loses the roll by two points or less, then both combatants inflict damage on each other. This tactic is often used by mindless undead, enraged or berserk creatures, creatures immune to the opponent's attack, heavily armored creatures, and those with a lot more hit points to spend than the opponent.
Guard: The combatant gains +2 to all combat rolls applying to it that round; if successful no damage is inflicted by either combatant.
Grapple: Instead of an armed attack, the combatant tries to grapple the opponent, and succeeds on a winning combat roll. Losing one roll results in being grabbed, two is taken down, and three is pinned. This could also be modified for climbing on huge opponents.
Surprise attack: The target does not apply its CR to the combat roll and suffers a -2 penalty, and inflicts no damage if it wins the roll.
Insubstantial attacker: An attacker able to ignore solid matter, such as a wraith, is unhindered by armor, and its damage is not reduced unless the armor is magical. Additionally, unless the opponent has a weapon capable of harming it, the monster attacks as if the opponent were unarmed. May be combined with Reckless Attack.
Seizing momentum: Winning two or more rounds consecutively gives the aggressor a +1 bonus to CR for each round after the first, to a maximum of +4. Resets when the defender wins a round, or when contact is broken or another combatant enters the fray on the defender's side.
It should also work well with Simple Combat Maneuvers.
One potential down side of this method is that, since normally only one combatant of a pair inflicts damage in a given round, fights tend to last a bit longer than they otherwise would. This only gets worse at higher levels. Equally matched opponents each have a 50% chance of dealing the other damage in each round, regardless of their levels. A pair of 10th level fighters do no more damage to each other on average than a pair of 1st level fighters, but they have a lot more hit points. In the rules as written they both hit each other more often, increasing their expected damage per round. I'm not sure whether or not this amounts to a significant problem. One possible solution is to flatten hit point totals for characters and monsters, to reflect the overall reduced damage potential. It might also be mitigated somewhat by making damage dice "explode" on a roll of max damage.
Another possible problem is magic armor. Since it no longer affects the chances to be hit, what does it do? Increasing DR even a point or two is a lot more powerful than a similar bonus to AC. One possibility is to make the magical bonus effective only against the pluses of magical weapons. Armor +1 counters sword +1, but does not affect a normal sword. Perhaps magical armor confers some protection from purely magical attacks such as spells or wands. As suggested above, magical armor protects against the attacks of creatures that ignore non-magical armor.
There are probably still some kinks and bugs I haven't addressed, but that's the rough idea.