D&D as written makes several assumptions about how a character's combat ability improves with experience:
- Ability to overcome a target's defenses in order to hit or threaten it, as expressed by THAC0, and thus cause damage, improves with experience.
- Ability to withstand damage caused by a successful attack, as expressed in hit points, improves with experience.
- Ability to defend against being hit or threatened and taking damage, as expressed in Armor Class, does not improve with experience.
- Defense is almost entirely based upon factors extrinsic to the character, such as armor and magic.
- The one intrinsic factor of defense, Dexterity, is static and does not change with experience.
Armor has a strong effect on a character's survivability in combat. In fact, Armor Class accounts for a much greater proportion of survivability than hit points. Most characters can't absorb more than a couple of successful sword attacks, so whether or not an attack hits is crucial. The inherent randomness of a 20-sided die provides for a lot of uncertainty. Anyone can die. Magical attacks that hit automatically are a great tactical advantage if used wisely.
As characters gain levels:
The relative importance of armor begins to diminish, but it remains an effective defense. Characters hit each other more often, but can also withstand more hits. As combatants will survive longer on average, combat duration increases, and thus each combatant will make more attack rolls. With more rolls, the average in any given combat is pulled closer to the theoretical average roll for the d20; thus a well armored character will likely take less damage over the course of a battle than a poorly armored one, but is also less likely to escape battle entirely unscathed than he was in low-level combat. Overall, combat becomes more predictable, but not ridiculously so. This is probably why mid-levels are such a popular "sweet spot." Players can put a little more trust in the law of averages, but unexpected things can still happen.
At high levels:
Armor becomes nearly irrelevant, as most characters can hit very low ACs with relative ease. Most attacks will hit and do damage, so surviving a combat depends heavily on hit points. The difference between weapon attacks that require an attack roll and spells that automatically hit is greatly diminished. Bolstering armor with gobs of magical bonuses ameliorates this only slightly, while rendering low level opponents virtually impotent, even in great numbers. Combat between evenly matched high level combatants becomes a highly predictable contest of attrition. Combat against hordes of lower level opponents becomes a highly predictable rout. To many players, I expect, neither scenario offers much excitement.
In my last post, I suggested slowing down the progression of the attack matrices as a way to hobble galloping combat inflation. This would extend the "sweet spot" to higher levels but ultimately it too would succumb to the sequence above. I also posited a system in which attack rolls are determined not by each combatant's level separately, but by comparing the two. Initially I included it almost as an afterthought, too radical a departure from traditional D&D to really take seriously, but the more I consider it, the more I'm intrigued by its possibilities. Next time, I'm going to go into greater detail on that system, and what combat would look like under it.