With all the pondering I've been doing lately about the abstraction of D&D combat, it's increasingly clear to me that one long-cherished rule just doesn't have any business being a standard part of the system. That rule is initiative.
Initiative in B/X is a pretty straightforward affair: roll 1d6 for each side, and the side rolling higher goes first. It's incredibly simple, and doesn't really slow down the proceedings that much. I'm guessing that someone found the idea of everybody on an entire side acting before everybody on the other side unrealistic, though, and so we have the optional individual initiative rules, wherein each player rolls separately, and possibly the DM rolls separately for each opponent as well. Still not "realistic" enough? Pile on dexterity bonuses, weapon speed factors from AD&D, and various other adjustments to simulate just exactly how quick any given combatant is on the attack.
The thing is, that entire line of reasoning, from the basic idea of an initiative mechanic through the proliferation of rules and adjustments of all sorts, is based on a premise that is antithetical to the assumptions of abstract combat. Knowing that one attack roll does not literally mean one
swing of a weapon, and knowing also that the damage inflicted by a
successful attack roll doesn't necessarily represent the effect of a
single thrust or slash, the idea of one side's entire attack routine
coming before another's is nonsensical. It's entirely possible, for instance, for a pair of fighters to "hit" each other and inflict 5 points of damage each through a back-and-forth exchange of 1 and 2 point jabs over the course of a round. Also, the initiative system often produces absurd or bizarre results, like one entire side standing still while every member of the other side completes its allotted movement unhindered, or an attacker covering 40 yards of distance and making a melee attack against a character who has a bow readied before the bowman can shoot.
So how does combat work without an initiative roll? Developing and systematizing some ideas from Charles of Spells and Steel (from his comment on this post):
1. DM states what each monster or opponent is doing in broad terms, i.e. closing to engage in melee, firing a missile weapon, tactical movement, casting a spell, taking cover, etc. If an enemy intends to use a power that wouldn't be readily obvious, such as an innate spell-like ability, the DM need not say so yet - only what's visible to the player characters is described.
2. Players state what their characters are doing, again in broad terms. Covert and "invisible" actions are stated here, though.
3. Everything takes place more or less simultaneously, but divided into three basic phases.
a. Ranged attacks, spells, and simple tactical actions by characters not yet engaged in melee, even if they will be in melee before the end of the round due to enemy actions. Simple tactical actions include drinking potions, activating magic items, casting "touch" spells on self or adjacent allies, etc.
b. Movement. Combatants moving toward each other meet and clash somewhere in the middle, rather than one side using its entire movement before the other.
c. Melee. Also, any spells or tactical actions performed after movement and/or by combatants who are currently being attacked in melee, whether or not they are making melee attacks themselves. For example, healing a front line fighter after moving up to him, drinking a potion or casting any spell while engaged in melee.
d. Actions that require attention and focus, such as lighting a torch, gathering loot, or binding wounds may be accomplished if the character was not attacked in melee at all, nor struck by a spell or missile attack. If any of those conditions obtain during the round, the action will generally fail.
Of course this sequence may be modified or ignored whenever circumstances dictate. For example, the players might decide to attack before the monsters do, and thus describe their actions first. A thief who has managed to sneak behind an enemy archer might make her melee attack before the archer's shot.
For the most part, every combatant will be able to take its action, even if it takes enough damage in the round to kill or incapacitate it. There are exceptions to this general principle; i.e., a fighter closing to make a melee attack who is brought to zero hp by missile fire, or a charging creature hit by a character with a spear set to receive the charge might not get to make their own attacks, because the other side clearly does act first in those cases.
Still, the good old initiative roll isn't quite useless. It's just used judiciously, for specific purposes. For example, it can be used to determine whether spells are spoiled by attacks. If one or more attack rolls hit a caster during the round in which a spell is cast, make an initiative roll between him and the opponent(s) after the fact to see if the spell went off before the caster was struck. (Since ranged and personal-effect spells are resolved in a phase before melee attacks, normally only missile fire or another ranged spell can threaten a spell if the caster isn't already in melee that round.) An initiative roll can also be used as a tiebreaker in cases where two or more characters or creatures are attempting some mutually exclusive goal, such as grabbing an item from a pile of treasure, and their movement rates and distance from the object suggest they both reach it at the same time.
The upsides to scrapping initiative as routine procedure, as I see them, are: Eliminating an incongruous and superfluous random roll each round, and allowing the freedom to resolve each round's actions in a sequence that makes intuitive and logical sense rather than being bound by said random roll.
The downsides: If you're accustomed to rolling an initiative die and then going around the table and rolling for each attack as it's declared, it essentially adds an extra phase to the round. The declaration of intent phase is separated from the rolling for resolution phase. Also, it eliminates the possibility of a party routing a group of monsters without being attacked in return by winning initiative in the first round. The monsters are going to get their licks in too, and this may increase the deadliness of combat, especially at low levels. Then again, one more reason to make combat a last resort isn't necessarily a bad thing.
A potentially interesting side effect is that healing spells and potions administered during a round in which the recipient takes enough damage to reduce it to zero hp or below may actually prevent death or incapacitation. A character reduced to 1 hp the previous round could pop a healing potion without worry that he'll be killed before he can drink it if his side loses initiative, because everything is simultaneous.