I've played in five ongoing AD&D 1E campaigns: three that I ran, and two that I played in, and all were from the years of 2011 and onward. I'm barely old enough to remember the 80s, let alone the earliest days of the hobby. But something about the archaic, byzantine nature of the old hardcovers always appealed to me, and that's why I designed my first campaign using the system. I had to figure out for myself what rules to use or ignore (I've never used weapons vs. armor class, weapon speed, psionics...), discern how magic users are supposed to gain new spells, and piece together rules that definitely do not make sense. Reading the Dragonsfoot forums helped a lot.
As others have said, I love the free-form nature of adding and removing rules, and I love the old-school feel of sword and sorcery, and the lack of long lists of skills and feats in favor of players just using their heads. Lots of things appeal to me about the system. I don't even mind THAC0! Many of my most cherished gaming memories come from the five campaigns I've run using the system. I've played a mixture of homebrew settings as well as famous published adventures like Ravenloft, Against the Giants, Barrier Peaks, and Demonweb Pits. I used to stan for 1E, and stan HARD.
But I feel a little differently now.
I recently started up a new campaign with players who are either new to D&D or are used to newer editions, and I've had a hard time rationalizing my old love for the system. I want them to have the freedom to make the characters they want to play, so it's hard to justify 1E's restrictions on races and classes. They're confused by all the different dice they have to roll for skill checks and initiative and thief skills, and they tend to forget if they have to roll high or low. And I can't really blame them! 1E in many ways just. doesn't. make. sense.
And what I've realized is that 5E can do almost all the things I like about 1E, but with the benefit of 40+ years of streamlining and playtesting. It's so refreshing to read these books that are well-organized and elegant, and pay tribute to the old-school games in ways that feel respectful but modernized. In the years I played 1E the most, before 5E's release, there wasn't a system that filled this niche for me. So now that it exists, I started questioning why I was so loyal to this old system, and what it offered that newer ones didn't.
Ultimately I came to the conclusion that I'd rather play the modern version. So I'm converting my megadungeon to 5E. But the 1E books stay on the shelf - lots of great information I can still glean from them, the DMG and the monster books in particular.