Three more things that make AD&D awesome...
(These can apply to both AD&D and AD&D 2e).
1. Proficiencies were there but not required.
I was satisfied the proficiency system in both AD&D 1st and 2nd editions. It was, however, quick and dirty. I didn't make much sense why characters did not get better as they progressed, but that's what you get with a system that was tacked-on.
The neat thing about the proficiency system is both editions is that you did not have to use it. It was available. But it was not an integral part of the system. The DM could use it or not.
When my brother introduced me to AD&D back in the late 1980s, we did not use the proficiency system. There was a lot for me to learn as a 10 year old. In fact, we never did use the proficiency system and still had a lot of fun.
When I started running games for AD&D 2e, I held off on the proficiency system until my players were comfortable with the basics of AD&D--combat, roleplaying, character skills (thief, spellcasting, etc). And we had fun. In fact, IIRC, using proficiencies helped "round out" a character but didn't detract anything from the fun.
2. Layering of systems and subsystems
Unlike in later editions, you can learn AD&D in bits and pieces and still have a good time. The Players Handbook had the basics on combat, roleplaying, and general advice. More advanced information resided in the DMG, such as the combat tables. (although I admit it would suck only having the PHB and MM in the days before the DMG came out).
The proficiency system is great example of this, along with the weapon bonuses versus different types of armor. They could be learned and used... or not.
As the rule supplements came out (like Unearthed Arcana, Dungeoneer's Survival Guide, etc), they could be integrated into the game or not.
The fact is, you didn't have to study the books in order to have a fun game or create an optimal character. It helped to study so you could get the rules down, but it wasn't required as a player. The burden of mastering the system relied on the DM (who should know the rules anyway).
In fact, AD&D almost geared itself for newbies to play. Each character class at first level had a small amount of skills to keep track of (yeah, the wizard only got one spell...). This made the game fairly simple to learn. The players didn't have to spend time mulling over their character sheets during play deciding what powers to use. Their attention was on the DM and what was going on in the game.
I cut my teeth playing a dwarven fighter. I've seen dozens of newbie players over the course of 1st and 2nd edition start by playing a fighter. It is the perfect class to begin AD&D. It has high hp, has a reasonable chance of getting decent armor, and it easily enables a player to learn the combat system.
Many of those players, of course, often moved on to a more specialized class. But they learned the combat system before moving on.
3. Lack of Ripple-Effect from altering the rules...
I came up with the "Ripple-Effect" term when 3e came out and I tried running it with a low-magic campaign world. Basically, if I altered one part of the system, it would cause "ripples" throughout the system. For example, cutting down on magic items would alter the CR of various monsters and NPCs. It would be difficult, for another example, to alter the skill system without effecting the rogue and the bard. Tinkering with feats might hose the fighter.
Rules altered in AD&D usually don't "ripple" out and effect other systems. For example, altering the proficiency system will not effect thieves skills. Another example, letting the poor 1st level wizard get bonus spells for high intelligence (like a cleric with high system) will not make him overshadow other classes (he still needs to conserve his spells and stay out of combat).
Or removing demi-human level limits won't effect the rules (thought it might affect player choices for races).
This lack of "ripple effect" enabled a DM to tailor AD&D to suit his desires and tastes without much difficultly.