Since it's late at night and I can't seem to sleep, I might just as well tell about a house rule I have no intention of implementing. I think it could truly warp the nature of combat, while the ramifications would be truly interesting ... but it would also be an enormous hassle.
Surely the gentle reader has noticed that, in cases where combat is about to occur in real life, the combatants do not rush into violence in the manner of D&D characters. Rather, they tend to hesitate and make feints, closing to combat not immediately, but after several failed attempts.
Remembering that a round in my world is 6 seconds, equivalent to the normally designated segment, I think the following rule could be incorporated. Suppose that in addition to ability, we interject a quality of 'bravery' into the proceedings - that being, the resolve on the part of the character that combat is necessary and must be entered into, in spite of the potential threat. I call it a thinking process, and therefore I suggest that the ability stat of wisdom be the crucial element here - to wit, that before a player can enter combat, a wisdom check must be accomplished and succeeded at.
Not merely an ordinary wisdom check, I think, but a wisdom check done at -4, or even -6, depending on how hard you want to make it for combat to actually occur. What a positive boon it would be to clerics! What a mess it would make of a party closing to combat, with one or two persons actually succeeding while the rest tried helplessly, round after round, to overcome their animal terror and successfully enter the fray! For fighters and thieves who ordinarily throw their worst stat into wisdom, what cowards they would be. If it wasn't for the sheer pain in the assedness of it, I would try this.
After all, if we measure most of their opponents by their intelligences (and not imposing the subtracting modifier - expect no fairness!), orcs remain fair combatants, goblins and kobalds turn to fearful entrants, and elves fearless killers. What a remarkable difference it would make to a great many creatures, since genius and extraordinary intelligence creatures could potentially butcher a whole party while they stubbornly blew their roles, standing in stock fear as their comrades were leisurely cut down.
I'm beginning to convince myself to try this.