D&D - Why 1e?

I was asked on Twitter recently what I liked about 1e AD&D. One answer to that can be found in my pinned tweet, it outlines all of the game mechanical features of AD&D that I like, why I think it is a well designed game:
Pinned Tweet
But outside of general game mechanics, here are the top 10 reasons I play 1e.

First things first though, I believe that other games can give you the sort of experience that 1e gives me, I don’t think is it unique in this regard. However, this is the experience the game gives me, and the reason I play it. I’m not suggesting other games don’t have some or all of these features. I’m just speaking to what I know can be done with 1e.

I play 1e AD&D in part as it is the first system I learned. I run other systems, but 1e is my home. Other than that, here’s why I prefer it as a system.

1. Modularity. Some games are built on a main mechanic or chassis, and if you muck with one thing the rest falls apart. 1e is uniquely customizable. For example, I’ve run 1e campaigns where we pretty much ignored alignment, and you only have to make minor adjustments to make that work. Conversely, if you want to go all out for alignment you can do that too. There is a reason why there are so many retroclones and versions of D&D. 2e is just a game designer’s set of house rules for 1e for example. 1e encourages you to make the game your own. Gygax warns against a bunch of things, but ultimately expects you to decide what to keep and what to leave out. So for example, many people don’t use the unarmed combat rules, or the psionics, and the game rolls forward just fine.  This design feature, with a main game filled with many sub-systems, is perfect for modularization. This is why everyone’s campaign is somewhat unique and one of the greatest strengths of the game. It also means you can run a lot of different kinds of game, high fantasy, sword and sorcery, gonzo sf-fantasy, whatever you like. This is another source of the retroclones and derivatives of 1e that exist, they are in many cases just reskinned D&D to create a particular style of play. 2. Death - PC’s will die in the game, it is deadly and meant to be so. This influences play, groups that hack and slash everything end up dead. The threat of death shapes the tone of the game to make it more exciting and more immersive. It also makes the achievements sweeter, and the mechanisms in the game for bringing characters back to life are expensive enough that they create great role-playing hooks. Some of the most memorable moments in the game come when a PC meets a glorious death. 3. Imbalance - D+D is not “balanced”, PCs will periodically encounter opponents and situations far beyond their capacity, they will have to flee or pursue non-combat options. This makes the game tactically opaque, you can’t be sure that what you meet will be beatable, so you have to play accordingly 4. Non-Combat Mechanics - D+D has encounter reaction rolls, rolls that can determine in a general way how NPCs or monsters will react to PC actions. As it is possible for an NPC or monster to react positively, combat is not the only option. Parley, negotiation, bluffing and deception are all options outside of combat, and as you play an ongoing campaign your players can figure this out and play accordingly. 
5. Experience - The experience point system in D&D gives the greatest rewards for loot (treasure and magic items) and the least rewards for slaying monsters. This leaves the players open to pursuing non-combat options and still advance in the game. It makes XP flexible and easy to account for 6. Light Roleplaying Mechanics - I like games with light mechanics for things that you (the player) already do well. So AD&D has alignment, race, skill, class, deity, loyalty and encounter reaction rolls as a guideline on role-playing, everything else is free form to the player to run as they like. I find this lends itself to characters developing in play, rather than playing to established character types. Less acting and more improvisation. 
7. Abstraction - Gygax liked to abstract things, damage, combat rounds, etc, etc, and the best part is if you don’t want to abstract things you can import systems to do so, this links to the modularizability of the game. So for example I know tons of people who use critical hit rules of some kind. Gygax didn’t like them, but they dovetail nicely with the game. Gygax abstracted a lot of things, you can make them more detailed if you want. In short I add some abstraction to areas he made too detailed, and take it away from areas where he made to abstract. 8. Teamwork & Resource Management - D+D is a collaborative game, if you cooperate with your fellow players you will be far more successful. This does not mean, however, that every character will have a chance to “shine” in every encounter. D&D is a marathon, not a sprint, the success of individual party members is shared by all. The XP system explicitly recognizes this. The general design features (spell casting, encumbrance, the imbalance of encounters) mean that resources must be managed or the characters will be challenged. I like this as it makes for a more strategic game.
9. Spells and Magic System- The spell and magic item system is hands down the best at giving lots of flava (verbal, somatic and material components), being flexible (casting spells you don’t know from scrolls, casting higher level spells than you can normally cast from scrolls) and if used as designed, meter’s out temporary magic and permanent magic reasonably. People who think AD&D is “Monty Haul” aren’t using the published tables and rules. 
10. Dice and Mathematics - I want some randomization in my game, and AD&D uses bounded randomness with multiple tables and sub-systems to determine things. This has three purposes:

1. It adds randomization, which keeps things fair, I have a lot of control over what happens, but eventually you bump up against a dice roll that I have no control over. It makes it harder for me to be biased, and gives the PCs a sense that the result is to some degree independent, which makes it seem more immersive, as the real world is somewhat independent. 1e has lots of subsystems you can use for this purpose. It also adds to the tactical opacity, if spells are randomized for example you don’t know what your opponent can do. This makes the game more exciting.
2. D&D’s combat system works by small cumulative additions to rolls, e.g. ability scores, magic bonuses, situational bonuses, this gives some room for tactical expertise. The weapon system, when used fully, is one of the best for this. Weapons vary with respect to 6 variables so you have meaningful choice, and the class system meters out weapons expertise and improvements in skill with weapons (with small iterative additions) in a way that makes it exciting and makes it differ by class. 
3. RRTEI - Roll Randomly and Explain It - AD&D has lots of tables, and rolling on them and having to interpret the result is an amazing source of inspiration for your game. Whether it be the encounter reaction table, the random encounter table, or even the magic item tables, you are forced to explain the result, and this is a source of inspiration. I run a sandbox game, so I need cues to help me improvise, 1e is stellar for this. Gygax gives you just enough to get the imagination going, but doesn’t detail everything. Some people don’t like this, I really do. It’s all dice driven. 

Those are my top 10, there are probably a few I’m missing, but these are the main reasons.