Which, I think, is part of what Celebrim and I find so confusing about the OSR movement.
Let me clarify exactly why I find the OSR/OSRIC etc. movement so confusing.
1) It's a movement to recreate a specific set of rules yet the fans of the movement when discussing why they are fond of the game almost never reference actual rules, but instead reference ideas about play, encounter design, campaign design, and so forth that are not aspects of the rules - for example challenge, skillful play, randomness, using propositions highly specific to and interactive with the fiction, etc. And I'm on board with most of that but don't understand why you'd need those rules to do it. My 3e campaign, the players unwisely interacted with some radioactive ooze, and now one player has a talon for a hand and other player's hippogriff steeds has a mutated foot with an alien mouth on it. I didn't and don't need 1e rules to have 1e feel. It feels like the same effect could be achieved by producing supplements for modern editions on old school play style, encounter design, campaign design and so forth.
2) Some of the most successful and prominent OSR games don't actually have that 1e feel either. Instead, they are more like what TSR games might have felt like if instead of Tracy Hickman advocating for the comic book code in future TSR products, Anton LaVey had been an employee and recommended doubling down on the occult scare for the publicity. Now I get there are people who hear something like that and go 'cewwwl', but I don't see why you need a 1e rule set for that either. Why is grimdark such a thing in the OSR community?
3) To the extent that OSR champions will talk about rules at all and advocate for rules, typically what you'll hear from them is that what they like about the rules is that they don't use them. That is to say they'll say that the great thing about the old rules is that there are no rules and they can just make things up. And ok, that may be a preference thing, but at the same time it's not a rules thing either. No rules set is comprehensive and regardless of which edition you are playing, if you are doing your job as a DM and if the players are doing theirs, then you'll find yourself outside the rules needing to make rulings. In my 3e game I found myself running a combat where the players were racing mounts down a city street next to a runaway carriage that was being attacked by wights, where like an old western movie I had heroes jumping off horses to get on the carriage and try to stop it. This is not a situation which is explicitly covered by the rules as written in 3e or 1e, and so regardless of which system I was using there would have been a lot of rulings involved. However, I certainly know which system has more support for this sort of free form play, and it's not 1e AD&D, and that is so obviously true that I really wonder whether the old school fans of free form play do really in fact have as game as free form as my 3e based game, or whether it's more like the play I remember of 30 years ago where we pretty much stayed in our lanes and delved in dungeons. I mean staying in the lanes and delving the dungeons seems to be what the OSR games are selling as a selling point...
As far as actual rules go, there are a few things I miss from the 1e era:
a) XP for g.p.: This is the biggest surprise to me of all because back as a DM in the AD&D era I hated XP for g.p. because of the constraints it put on campaign design. The rules strongly encouraged leveling by getting rich, which meant that the PC's coffers tended to be overflowing or the PC's tended to die from the grind. To me it was like the training rules. I got why they worked that way, but that didn't stop me from tossing them out the window at the first opportunity because they got in the way of a fun story. But, now that they are gone and after some years of reflection I do miss them at times. Or rather, what I really miss is players being excited about finding loot. By and large, players in my game don't care about gold. They care about magic items, but gold because it isn't readily fungible to magic items (unlike some 3e games I presume) isn't something that they care much about. And that feels like a bit of a loss both in realism, game play, and fun.
b) Weapon vs. AC modifiers: The rule that I missed almost immediately in 3e play is one which I doubt any OSR games actually implement and which few tables at the time used. But if it wouldn't make the game more complex than it is, I'd bring it back in a heart beat.
c) Exponential XP progression: Of the things I miss this is the one which I'm most likely to actually incorporate in my rules set in the future. I'm convinced that it is superior to linear XP progression to level having played with both. However, changing the rules so that exponential will work well (especially in absence of the fudge factor of treasure for XP) will be a huge undertaking and so far I haven't attempted it. I miss the way exponential XP supported henchmen, supported starting new characters from 1st level if you wanted to go that route, and the way it created natural demographics if you made assumptions about NPC's gaining XP over time.
But that is literally it. Everything else that I've got now is the same 1e underlying 6 attribute class based chargen with the strong D20 fortune engine, only the chasis that has been built around that classic engine is so much better in every respect to what was built around it back in the day. It's clearer. It's fairer. It's more balanced. It handles difficulty cleanly. It supports more open play better. It's more complete. It supports more diverse character concepts. And perhaps most of all, it's got 10 years of house rules built around it to make it play the way I want it to play.
One of the few AD&D DM's I encountered that I thought I understood was one like me that I had built up a body of house rules and porting away from that and what he was comfortable with he said was too much work. But then, at the time he wasn't part of OSR. He was just a DM that had been running games the same way for decades. Only since then, he's taken up LotFP, and I'm like, "What?!?!? Why? I thought you were completely comfortable with your house rules and couldn't change."
At some level I think I do get it: "How you think about a game and how you prepare to play is at least as important as the rules." And I think for most people they have to have a rules change in order to change how they think about the game and the habits that they bring to the table. And that sort of makes sense that your habits would get attached to a particular context. But then I get in a discussion with OSR people about the rules, and they are all like, "If you don't like the rules, it's because you aren't a capable enough DM/player. Using the old rules requires skill and imagination, you see." and it's all so totally not self-aware.
