One of my biggest issues with playing DND is that I never fully understood the rules. I'd play with people who had been playing for years, and they didn't explain things very well, and that made it hard to play. Hopefully, this will help with that.
I played from the early 80s through early 90s. Mostly AD&D 1e but earlier on the red/blue boxes and later on 2e.
Recently I've taken to reading r/adnd for nostalgia reasons. One thing become abundantly clear real fast, no one I ever played with ever truly understood the rules. Even the "rules lawyers" among us. And I played with a large variety of people from different friend groups, to different game shops, and even some smaller cons.
We understood the key details for the parts we actually used, but we weren't intentionally avoiding the rest, we just didn't understand that they existed. There's just so much minutia in those rule books.
This also makes me chuckle when I see newer players come into r/adnd as part of the OSR movement. Because they *do* seem to assume that all of these rules were commonly applied. But my anecdata would say otherwise. I originally assumed that these newcomers to the old rules would be playing a game I found alien as they'd be bringing in newer sensibilities, but instead I suspect I'd find it alien as they're more likely to be sticklers for the full ruleset!
Like yourself, encumbrance was one I rarely saw used. There was usually a rough sense of "too much" but otherwise no one cared.
Shadowdark does not only have much simpler (and fewer) rules, there's also a lot less world building. This encourages the DM and the players to create their own fantasies, rather than adhering to the races described in the (MASSIVE) DnD manual.
D&D has a strong narrative aspect when you look at the published adventure modules. There are usually plenty of characters to interact with in some way or another and some quests can be solved entirely by following the breadcrumbs offered up through them. But the DM needs to role-play all of these characters and do a lot of improv to make this work. This isn't so easy.
Also, combat in D&D is a slog. Whereas turn taking outside combat is rather fast and loose, the game turns into this enormous ceremony once the words "roll initiative" are spoken. The effect is that combat can take up a lot of playtime relative to the non-combat role playing, while often also leading to less overall quest progress per time.
DM: "Umm... not very good..." (became a running joke)
Whereas today's game is far more complicated rules-wise by most measures yet it tends to be more storytelling & *role* playing focused: flower-y, superhero-y, high fantasy
Most of the early old-school stuff was way too deadly for players to be murder hobos or try to solve everything with combat - if you went into Caverns of Thracia at level 2 as a murder hobo you're just going to die over and over and over again. It'll be endless TPKs. Right now I'm two years into DM'ing an Arden Vuul campaign, running a mix of OSE (Streamlined B/X) and OSRIC (ADD 1e) rules, and it's really only the past 6 months or so that my players have felt comfortable engaging in regular combat - before then they might have spent a whole session or two trying to stack up every advantage they could because they never wanted to be in a fair fight.
And from my experience with a whole lot of OSR play over the past 6-7 years is that this is the sort of feel most OSR players are after. They're not wanting to play late ADD 2e, Dragonlance era, where the shift to the more heroic play started happening - they want to have to think and outsmart things. Faction interaction was also huge in the more sandbox environments, and that was where most of the roleplaying occurred then, and occurs now in the games I run. The players RP a bit with each other, but not as much.
Modern D&D is a kitchen sink approach that tries to solve every possible playstyle, and that makes it popular and reasonably good at most anything people want to do with it. But I don't know that there's any facet of it that it does as well as other systems.
Yeah this is kind of my point, that I think a lot of the contemporary play style is cultural and not ruleset driven. And thus I'm skeptical that merely doing something playing 1e AD&D is going to feel exactly the same as it did 40 years ago. That said, I may be overstating how typical this is in the modern game, I haven't played in 30-ish years, my take is driven by observation purely.
And also, even back then some of the more modern improv-y play style existed, it just wasn't the norm. I remember when my main play group had a session with one member's brother & friends and there was a very clear culture mismatch from the start. They were acting, with voices and all of that. We ... did not. To each their own but combining the two didn't work.
D&D is better as a video game. Try Baldur's gate. It has the side benefit of teaching you the rules if you ever want to jump in to a local game
It's probably way different than you expect (and will be different between DMs).
To some extent, this is weirdly a good thing: if you want strictly enforced rules, you may just want to play a videogame instead. D&D succeeds best as a social lubricant enabling a framework in which social gaming (roleplaying) can happen to be "fun". Rarely is strictly following rules "fun", especially socially with friends; the rules in D&D are meant to be guideposts and tools for enough structure that people that want structure find comfort and enough flexibility that "fun" isn't lost in the process.
Which is a long way to say that you probably aren't going to learn the right lessons from a well fuzzed computer spec of the rules, you probably are going to learn more lessons asking the people you play with what rules they find important, to explain things you feel you don't understand, and to suggest which chapters in which books to try to read to best improve your understanding for that group. At the end of the day, if the table seems too hard to play at you might also just be playing with the wrong group, especially if you aren't having fun.