A common complaint I hear about tabletop characters in popular but poorly-balanced games is that defenses never quite function harmoniously with destructive potential. Anyone who's been on an RP board ever is well aware that this problem is not solely the territory of tabletop games ("Twirling with artistic grace, she swings her crimson lightsaber at the Jedi's-" "I dodge."), so feel free to read along even if you've never played a pen and paper game in your life. Or, y'know. If you just want to. That's cool. It's cool to have fans.

AC and DR - Evasion vs Mitigation
In ye olde tabletop Dungeons and Dragons, the idea that someone was attempting to evade your attacks birthed the now ubiquitous d20 system. Of course your target doesn't want to be hit. That's why, if you both utterly suck at attacking and defending against attacks, you have a 55% chance to actually connect with a blow. From there, modifiers are added to both your defense and your accuracy. A trained warrior will of course have a better chance to hit an opponent who's attempting to defend itself, and an armored knight will have a better chance to not be hit at all. Suspension of disbelief achieved, score one for D&D.

At least for a couple of generations. More recently, in D&D 3e and 4e, the creators seem to have wised up to the fact that gamers were not content with the idea that armor only assisted your evasion rather than your durability. Armor Class was a "paper defense," enhancing your evasion rather than your ability to absorb damage. The two sound similar at a glance, but anyone who's played mid- to high-level D&D can tell you that AC frequently means sod-all against high-level threats...many of whom don't even need to hit your AC in order to do damage.

In order to address this issue, 3e and 4e (and 5e, to a simpler and thus better extent) used an ability called "resistance." A character with damage resistance 10/slashing would ignore a portion of damage (the first ten points) from any non-magical attack that did not deal slashing damage. In 4e, this would be Resistance: Slashing 10, and would function even against magical slashing damage. Another good solution. You now have genuine durability against certain kinds of attacks, especially if you could find the coveted DR/- and Resist: All, both of which did exactly what they sound like they do.

Hardness vs Soak - Immunity vs Mitigation
This sounds like a simple yet sophisticated solution, but D&D was one of the slower game systems to pick up on it. In older games that have run for a comparative amount of time, such as White Wolf's flagship franchise Vampire: The Masquerade, the idea of armor contributing to durability rather than evasion has been a mainstay for the entire run of the system. Rather than adding to some kind of "you can't hit me" value, each level of armor provides a corresponding amount of "soak."

Now soak is one of my favorite mechanics ever, and it's only possible due to how White Wolf does their dice. While rolling damage, a VTM player will have a pool of d10 dice to roll. Each individual die that rolls a given value or higher is a "success," and the entity that's been struck has to take a point of damage from it. The defender then rolls their soak to counter the attacker's damage. Each success on a soak die subtracts a point of damage taken. In this way, it is ALWAYS possible to damage ANY foe, though the entire thing is based on luck...which isn't at all uncommon for tabletop games.

The later White Wolf game "Exalted" introduced the concept of hardness to supplement mitigation. Exalted itself is a good example of how to handle defenses in theory (though in practice it was grievously imbalanced) as a given character had a controllable defense value (evasion) and soak (mitigation) in order to simulate both the willingness not to get hit and the ability to mitigate that hit. Hardness, on the other hand, was the idea that some attacks were simply too weak to damage you at all.

In practice, hardness functioned very similarly to D&D's DR ability. If an attack did equal or less raw damage than your hardness rating, it simply failed to pierce your armor/break your hide/penetrate your magic shields/whatever. Hardness was very rare and often a very small number. Normally, hardness had no effect at all on real battles between two powerful characters, as both were easily bypassing each other's hardness with each attack. However, it was invaluable in battles against a number of weaker enemies, where an individual attack often didn't do nearly enough damage to break the armor at all.

Applied to RP - Evasion vs Mitigation vs Immunity
There's a lot we can learn from this. Most characters on this site have the ability to avoid an attack, but evasion is random and erratic. It's not just possible but inevitable that someone with a 50 AC will get hit with a natural twenty on an attack roll, and not even the best parry defense is unbreakable. Evasion is the first defense to take into account, but also the least reliable of the bunch. Sure, sometimes you won't get hit at all, but it's bound to fail you sooner or later.

Many players confuse mitigation for immunity. When you have armor on your body, you may be immune to some attacks. Notably, you're probably immune to attacks that wouldn't hurt you that badly in the first place. However, armor does not render you immune to everything simply by merit of being armor. Class 10 phrik armor will still dent and bend when pelted with an E-web mounted gun. Your armor should be treated as mitigation, rather than a hard shell that allows you to ignore all incoming damage with no adverse effects.

This has two really awesome side-effects. First, it means that people won't be so blasted annoyed with you no-selling their attacks all the time. Remember, there are two people in your duel. Your opponent likes to feel just as badass as you, and it's really hard to do that when you keep ignoring their attacks because "class 10 lol." Next, it lets you look like a complete badass. There is nothing cooler than being punched through a wall and struggling to your feet, your own blood running down the damaged portions of your hardened steel pantaloons.

I admit that I just wanted to write "pantaloons."