While the point of D&D is to accomplish some goal by performing the right actions, the mechanics of D&D can be seen as a way of acheiving a multitude of states, or conditions.
To exist in certain conditions, particularly the conditions "alive" and "able to take actions freely," there are myriad defenses in place; these include hp, saves, AC, and ability scores.
HP and ability scores are very basic defenses that protect against the dead and disabled conditions, and the helpless condition, respectively. Saves are defenses against a host of conditions, as well as a defense for the other defenses, at times, in the case of ability damage and HP loss for blasting spells. AC is more of a defense against needing to worry about any of the above, and is often skipped by spells; as such, I will not discuss it at length.
What they Are:
Now is where I make my first point: While conditions are binary in nature, the defenses against them are not necessarily. These non-binary defenses are gradual defenses, as I have come to call them.
Let's start with an example of a binary condition caused by a binary defense, for simplicity's sake. The condition "blind" might be caused by the blindness spell; you are either "blind" or "not blind," that's a binary condition (as all are). You make a Will Save to avoid becoming blind. Now, if you fail your Will Save, you become blind; however, if you succeed, there are no ramifications besides an opponent's lost spell slot. A Will Save is a binary defense.
HP are the classic gradual defense in 3.5. HP defend against the conditions "dead" and "disabled." You are either dead, disabled, or neither, these are binary conditions (as all are). However, HP itself is a gradual defense: If your HP is high enough, an attack will not kill/disable you; however, the next attack is more likely to kill you because your HP are fewer, and your defense is consequently lower.
In short, the difference is this: gradual defenses become weaker as they are assaulted, while binary defenses do not.
While taking ability damage hurts your power, no conditions are truly gained until they reach zero (unless your STR lowers enough to increase the severity of your load). As such, they are gradual defenses.
HP and ability scores are gradual defenses, and Saves and AC are binary defenses.
Why You Care
It is my firm belief that the primary imbalance of D&D comes from a twofold error by the designers based on a single problem: binary defenses inflict status effects immediately, or they don't; as such, attacks on binary defenses are universally more dangerous.
Keep that in mind.
The two errors made by WotC are these: Magic attacks binary defenses more often than gradual defenses, and the only time mundanes attack binary defenses, they have relatively weak conditions to inflict.
This means that magic as a whole has more chances to inflict the conditions of dazed, stunned, paralyzed, confused, dazzled, prone, cursed, charmed, dominated, and even dead, all of this without having to worry about how strong their oponent is. Each spell carries the full chance to inflict one of these conditions, based only on their opponents' saves. As long as they have enough spells to throw, they will eventually get one in.
What do mundanes get? They get to occasionally inflict the conditions of prone, grappled, stunned, and sometimes dead (in the case of coup de grace), if the conditions are right, and if they specialize enough to do so successfully. A lot of those even allow opposed checks! (Why don't casters need to make opposed checks to inflict conditions?) Otherwise, they are forced almost unilaterally to assault only gradual defenses. This means that each of their attacks carries less weight than most non-blasting spells, because the condition of "dead" is harder to reach in principle than any spell-caused condition.
That means that, action-by-action, mundanes are mechanically less effective than casters from the start.
By making casters bypass gradual defenses, WotC made casters pack more punch than mundanes with every attack. The only solutions to this would be either making mundanes attack binary defenses with more potent conditions more often (as Tome of Battle took a step towards doing), or to make casters attack gradual defenses more often (as 4E did).
Either way, you have helped level the playing field. However, since their are literally thousands of spells and only one little book full of maneuvers, neither solution has been fully realized, and we can see the effects of that in play with any fairly optimized game past 10th level--any game past 15th level without maneuvers.
D&D is inherently imbalanced, in my opinion, as expressed above. I welcome your opinions, however, on the issue.
Is D&D balanced to you? Am I giving ToB less credit than it deserves? Am I giving magic too much credit?
What other binary and gradual defenses have you seen, from any game? Do they work well? I am always interested in experiencing new things, new takes on familiar ideas.
Give me your thoughts!