Oangband

There are many changes to the game from vanilla Angband, but the most noticable change is to the combat system.

In Angband, the combat system worked like this. You have a certain number of blows per round, more for lighter weapons, less for heavier. Each hit with the weapon does the listed dice damage, plus any modifiers. For example, a whip does 1d5 damage. A whip (+8, +8) adds 8 to your to-hit roll, and to your damage roll, hence causing 1d5+8 damage. Since after a time, combat bonuses become so high that damage dice is essentially irrelevant, the best strategy for melee combat is to equip your character with the lightest, most heavily enchanted weapon possible, to take advantage of multiplying the damage modifier by the number of blows. (This makes a whip +9 with a 1d5 damage die that you can strike 6 blows per round with substantually more powerful than a lance +9, with a 3d8 damage die, but with maybe 3 blows per round.)

Oangband fixes the logical inconsistency of whips and daggers being the most lethal weapons in the game by changing the way magical bonuses to weaponry enhances damage. In Oangband, each character has a "Deadliness" score, represented as a percentile. This rating can be increased through a high strength score, and also by wielding magical weaponry. Each point of the magical bonus adds to the characters deadliness rating. Every 100 points of deadliness doubles the value of the dice rolled. So a whip, wielded by a character with a deadliness rating of 100 causes 1d10 points of damage, rather than 1d5. This system places a premium on the heavier weapons, since they use more dice to determine damage. (Whip: 1d5, Broadsword: 2d5, Lance: 3d8) Therefore, the damage for these weapons wielded by a character with a deadliness rating of 100 would be 1d10, 2d10, and 3d16, respectively. The effectiveness stems from being able to apply the deadliness bonus multiple times.

Oangband also has many other changes made to it, including to the classes, races, other aspects of the combat system, and also the magic system, but the completely original combat system among Angband and it's variants causes it to stand out more than any other change.