- Actions are declared in order from lowest HD/lvl to highest.
- Actions are resolved in order from the highest HD/lvl to the lowest.
- Characters with the same HD/lvl act simultaneously.
- Any character/monster may add a d6 roll to their initiative if they wish, but if they roll a 1 that action is fumbled.
Using experience level to determine initiative makes sense to me. Those better trained or more experienced will react more instinctively in combat. They will have faster reaction times because they've been in similar situations before. Where less experienced characters will be actively looking for an opening in their enemies defense, higher level characters will strike reflexively as soon as the opportunity presents itself. More experienced warriors will also be more adept at using feints and other misdirection to create openings and opportunity for attack.
The optional d6 roll is there because I know that sometimes in combat a person will sacrifice accuracy in order to gain speed. Mistakes can happen when you rush yourself. Including the fumble chance within the initiative roll allows blunders for actions without an attack roll, like falling down as you try to run away, breaking the bottle as you rush to use the healing potion, or accidentally shooting yourself in the foot as you try to fast draw in a duel.
It's not a perfect system, I know. HD of monsters often reflect size as much as combat skill. Maybe they should get a -2 to their initiative per size category above medium. I might also adjust non-humanoid initiative up or down for other reasons, but I find that I can estimate this in my head near instantly. It was always counting all the modifiers for all the PCs that seemed to slow us down.
Modifiers for Dexterity, weapon speed, and casting time all make sense to me, but I don't really feel that they produce enough fun to justify the time spent tallying and comparing these numbers that can change every round. Having a flat number with an optional d6 added is far smoother.
Arguments can also be made for a Dex as initiative system, but personally I think experience would effect initiative more than hand-eye coordination. Besides, a monsters' HD is easier to find than their Dex.