I started with BECMI and then discovered 2E. 2E is my most beloved edition of the game.
One thing that old D&D did that d20 D&D gets wrong IMO is saving throws: saves in old D&D were a function of the character, not the targeting effect, and could therefore be really useful in balancing things out. Fighters, for example, had great saving throws which kept them relevant even as the casters leveled up. To a lesser extent, NWPs did the same thing regarding skills: it was about how talented the character was, not how difficult the thing was.
One thing that old D&D did that d20 D&D gets wrong IMO is saving throws: saves in old D&D were a function of the character, not the targeting effect, and could therefore be really useful in balancing things out. Fighters, for example, had great saving throws which kept them relevant even as the casters leveled up. To a lesser extent, NWPs did the same thing regarding skills: it was about how talented the character was, not how difficult the thing was.