I have been wanting to post a thread like this for a while, but just can't find the hour or so it'll take me to put all of my thoughts down in a coherent fashion.
The current combat system has multiple shortcomings that all relate to the value of crits, rather than the action dice themselves, and the problem carries through the whole game.
What an action die is supposed to do, IMO, is enable coolness. If you come up with the perfect outcome for this scene, or a really cool action that would be a shame not to pull off, the action dice should let you buy your way to the result (if you have enough). What action dice do de facto, in many cases, is pay for the coolness once you've rolled the crit. I think the crit is too powerful, and too limited. I'd rather see all critical results toned down a bit, but give players the ability to roll AD's and add them to the original result til the dice total reaches the crit threshold.
In combat, the current incarnation of crits does two things, IMO:
1) it can lead to an anticlimactic combat result by seeing the main villain one-shotted
2) it makes it really hard to reach the value of a lethal attack with any other action, making the fights less interesting. Every roll for a lethal attack gives you a chance for a crit which can end the fight, and even if you don't meet crit value, you still take off hit points, bringing the fight closer to an end. Almost all other actions have less direct effects: either you deal alternative damage, you buff yourself (but only by a relative small bit), or you debuff the opponent or take away actions (grapple, trip, disarm). To most players, all of these are less powerful than the chance of killing something outright, so most turns go: step up, hack, hack.
One of my simplest suggestions were:
Action dice are for actions!
Action dice may only be used to increase skill checks, attack rolls, and saving throws, but cannot be used to boost damage rolls. Likely has to go with an overall decrease in vitality.
An activated crit doubles the damage done by the attack. The doubling only applies to weapon damage and strength and magical bonuses, but not to sneak attack damage.
Buy your way to success
AD spent on an attack check or skill check are added to the d20 result when determining if a threat is scored.
I think vitality should always be matched to the desired combat length. It would now be a more reliable timer and gives a better control. I do like having both vitality and wounds: a set of hit points that regenerate quickly and one set that regenerates slowly. It allows heroes to get back into the action quicker, rather than having to sit around and heal, and I've never been a fan of the D&D "grind your characters down with warm-up encounters so they have fewer resources for the final fight" deal.
I think I'd like an SIFRP kind of wound even better: instead of taking the damage, you take a wound, which gives a penalty to your rolls. You can either, like SIFRP, say the character is out of the fight when his active score drops to 0 from the cumulative penalties, or you assign a capacity to each character based on class and CON. That way, you can start each fight with full hp (or vitality), but still have the lingering effects of a more serious injury.