D20’s work well for a combat heavy system with multi-turn resolutions because the multiple rolls will create their own bell curves, but the odds of individual rolls are easy for even casuals to grok.
Just a casual here...can you explain to me how linear d20 results turn into bell curves because you have multiple rounds?
EDIT: I should add, the reason I ask is I always assumed d20 remained linear.
I was just assuming he meant that if resolution requires many rolls, the overall results will tend to cluster toward the middle, just like a single roll with many dice. Not sure of the details of how the rolls are used though.
Pretty much what Trond said. If you view an overall battle as a single task, then unless the contest is over with a single die roll the results over the course of a battle and adventuring day will average out based on your level of ability.
Basically, you will have a bell-curved result of "damage dealt per round vs. a given AC" in D&D.
For example, you have a +5 to hit and deal 1d8+3 damage vs. some orcs with AC 17 and 12 hp each. Now, you could get lucky and roll a 11+ for every attack or get a critical that deals 12+ damage in single swing... or you could whiff three times in a row or roll 1's on damage die and have to whack one of the orcs three times to make them drop.
The results of each swing will vary greatly in outcome, but after a few dozen orcs you'll find that your overall results are that you need about four attacks to dispatch an orc.
Similarly, how often the orcs hit you and how much damage they do per hit will, over the course of many encounters, average out such that you can guess about how much damage you'll have taken after a fight with X orcs.
There will be variances... sometimes your dice are on fire and you drop them in two-thirds of the usual time... sometimes they're cold and it takes you twice as long as you'd think (meaning you take more damage in the meantime)... but overall you'll get a fairly bell-curved distribution of results for your combats with those orcs.
And that's why D&D's combat system, despite using a 1d20 instead of say, 3d6 that produces a bell curve all on its own, still feels okay and like its not a completely random distribution outcomes. Any one individual attack is, but you generally need many attacks to resolve things so overall the result is a bell curve distribution.