Things I like about OD&D

 The simplicity of the numbers. 100 xp per hit die, d6 hp per hit die, all weapons do d6 damage.

Armor Class is entirely dependent on worn armor, except in the case of high dexterity which gives a benefit of at most +1.  This is only an issue with rising THAC0, but for PCs THAC0 rises slowly. Monster THAC0 rises by hit die, so it can be turned into a simple formula - monster THAC0 = 19 - hit die. As PCs gain levels, they gain more hp, so that their greater vulnerability to getting hit is commensurate with their greater ability to take damage.

All of this might seem too simple, but it exists within an elegant ecosystem where one mechanic flows into another. AD&D dissociated all these mechanics from each other, breaking them in the process. Variable hit die and variable weapon damage, new xp determination formulae, THAC0 and AC are no longer symmetric, and on. Maybe there was a wisdom in doing so that was born from actual play, that is otherwise not readily apparent.

AD&D adds a lot of cruft that I'm not a fan of. The Half-Elf and Half-Orc races, the special powers given to Paladin and Ranger classes, the Monk class, numerical modifiers and bonuses to everything.. The Psionic appendix and chapters might as well not exist to me. Yes technically all of this came from OD&D supplements, but they are mercifully not part of the core game. There is an assumption in AD&D that everything must have a mechanic attached and if that mechanic does not confer a numerical bonus, then it is not worth using. That may not be what the designers intended but a lot of groups played it that way, and I can see where they're coming from. This is a criticism usually leveled at 3e or beyond, but in AD&D 1e I see the genesis of all the sins of 2e, 3e, 4e and 5e. It's really hard to enjoy the game knowing that a few simple spells or magic items could completely break the game, or a player who decided to make a certain build could do it at will.   It is the least balanced edition of D&D, and that's saying something. There is a necessity for even low-to-mid level PCs of requiring +stat bonus magic items just to stay alive.

 I originally looked at BECMI to alleviate my issues with AD&D, but there I ran into a new problem - no one wanted to play. The general atmosphere is that "Basic" D&D is not interesting, and even OSR players prefer newer OSR games that are more horror or gonzo fantasy inspired. I assumed that with the current popularity of OSE that I would get more people to play BECMI D&D, but that hasn't been the case.  BECMI also has more haters, where B/X does not, and that I don’t understand.

 A lot of people who play OD&D like to use it as a springboard to wild and weird stories, the gonzo fantasy that the OSR scene is enamored with. I don't play that way. I am a very minimalist gamer and I enjoy low magic, low fantasy worlds with a simple collection of items and classes. Even my monsters are more naturalistic than supernatural.