It is amazing how everyone says old D&D was impossible to learn and GMing was even worse, yet most of us it managed it at 10 or so.
I'm split here. You're completely spot on that we did learn to play and had fun but also overlooking some major issues. Specifically:
1) Every single group you met played D&D differently, unless they, like, had some of the same members - and even then they often played it wildly differently. Rules were constantly ignored, entire notepads of rules were added, rules were wildly and insanely misunderstood on a routine basis and so on. I'm not saying we didn't have fun, but let's not pretend everyone "understood" the rules - they didn't. The house rules often showed this really well - so many games had house rules for stuff already in the rules but where they'd just never read those rules or misunderstood them. And a lot of the blame for that goes on how not-great a lot of the DMing-related writing was, even in 2E (but particularly in 1E).
2) The average standard of DMing in say, 1989, or 1995, or 2005 was heinously lower, in every possible regard, than it is today. This isn't a single thing that isn't, on average, massively better done today by DMs than it was back then. Many DMs were just terrible tyrants who ran awful games and were not fun to play with. Some, miraculously, you could play with today, and they would seem like a typical, normal DM - I got very, very lucky that the woman who taught me D&D was one of those - decades before her time. But most of the 1E DMs I met were petulant, incompetent, had terrible grasps on even 1E's rules and tended to run games that were either death-fests or the un-fun kind of monty haul. Again not everyone was like that, but so many were.
This was very noticeable on the early internet too, because people discussed good DMing, and like half the posters were like, angrily opposed to the basic standards of DMing now, stuff really 98% of people on this board would just assume as a baseline, like treating your players with respect, and expecting them to do the same, or treating players consistently, not playing favourites. I remember arguments from the 1990s where multiple people were arguing that playing favourites was a good thing, for example, just mind-blowing in retrospect. Hell, just look at Gygax's own book, which he later disavowed, Role-Playing Mastery, which was absolutely packed full of terrible advice (again so terrible that even he disavowed it), which again, even when I read it, probably in like 1990 or 1991, I was struck by how bad it was - but on the internet a few years later I saw how many DMs subscribed to that sort of thinking - and it was tons!
Do any now? Not really. You don't see stuff like that anymore. But it used to be routine.
