Gelatinous Icosahedron

 Combat offense in D&D is modeled with attack rolls, using d20, modified by class and level plus ability score modifiers or by creature Hit Dice, in either case representing some combination of skill and physical power, and by a damage roll, generally representing the amount of force brought to bear on a successful attack. It can be a wonky system, and sometimes counter to our intuitions of how combat and bodily injury actually work, but it serves its purpose well enough in the game. Defense in D&D combat can be even wonkier and less intuitively satisfying. Like offense, it's broken down into two primary components, Armor Class and hit points. Both, conceptually, serve the same broad function, which is to represent the avoidance or reduction in severity of bodily injury, but they do so in ways which are mathematically distinct and very different intuitively.  Armor Class is the more intuitively comprehensible of the two, though it itself represents two semi-distinct co...

 Way, way back in the long-ago days of the mid-80s, I was introduced to D&D through a copy of the Moldvay Basic Rules and the accompanying module B2: The Keep on the Borderlands. Before I ever worked up the courage to ask my siblings if they might like to play this game, I had read both books cover-to-cover at least a dozen times and thumbed through the most interesting bits still more, and they left some impressions. One of those impressions was a sense of scale regarding hit points. The average starting player character might have 7 or 8 if he were lucky, but maybe as few as 1 or 2 (though I did take to heart the suggestion that players be allowed to re-roll at 1st level only if the dice came up 1 or 2.) Lots of 1st-level NPCs had hit points below average for their class's Hit Die -- I recall plenty of Keep guards manning the walls with 3 or 4 hp. NPCs of 2nd level seemed quite tough with 7-10 hp or so, and those of higher levels with hp totals in the teens were real badasse...

 You thought I was gonna get all political, didn't you? But no, I just want to talk about Constitution in basic D&D, and how it's at least a little bit broken. Constitution, as written in B/X and BECMI D&D, has a much larger impact, for good or ill, than other ability scores. These editions use a modifier scale from -3 to +3. Most modifiers are applied to a d20 roll, meaning they can have, at most, an impact equal to 15% of the die's maximum. That makes a + or -3 modifier nothing to sneeze at, but it's not big enough to render a character either unplayable or insuperable.  Constitution, though, tacks that -3 to +3 modifier onto rolls of d4, d6, or d8. Despite the huge disproportion, nobody ever complains about piling a +3 on top of a d4 at each level, even though it more than doubles the average hit points of such a character (2.5 base, 5.5 after +3 modifier) and it's not really a huge deal to have an exceptionally hale and hearty magic-user have more hit po...