The two main ones that come to mind: In first edition the Armor Class went from 10 downwards eg AC -2 was awesome. And you had to look up tables (before 2nd ed

The two main ones that come to mind: In first edition the Armor Class went from 10 downwards eg AC -2 was awesome. And you had to look up tables (before 2nd edition added the slightly simpler THAC0). That was changed such that numbers always go up and similarly, higher dice rolls are always better (in some circumstances in 1e you wanted lower numbers). The concept of advantage or disadvantage. Sometimes you'd have to do a bunch of calculations eg -4 for being invisible, +2 for this, -1 for that, etc. Now, you just figure out whether someone has an advantage or disadvantage. If neither, it's a normal dice roll. If advantage, roll 2 dice and take the highest number. If disadvantage, roll 2 dice and take the lower.