I've been enjoying "OSR" (Old School Renaissance) style D&D for a few months now. Here's the things I think make it fun:
-The rules tend to be much lighter. This creates a blank space that is filled by improvisation and DM rulings. You're surrounded by skeletons? You can say "I want to try a spinning attack that hits all of them." The DM is not going to say "Nope, you need a specific feat to do that", they're gonna go "You can try..." and invent a mechanic on the spot.
-The game style is challenge based, and not "balanced". If you go into the dangerous area, you are taking a risk. But if you're careful, and clever about approaching that risk, you can mitigate it and win big. Playing safe levels you up slowly. Playing rough makes you feel like you earned your XP, found something you weren't expected to get.
-Oldschool games favor "emergent" plot. The story is created by the events and choices at the table, story is not imposed on the players by the unseen narrator. The DM doesn't have a script, they're not pushing you towards the next pre-written plot point.. they get to be surprised by what happens too.
-High stakes make the dice interesting. Epic loot can be found. Characters die. Yeah, it can suck to lose a character, but you'll love your next one even better. And when a character has survived a long time, they feel very 'storied'. OSR games don't frontload character-investment... they build emotional attachment over time. In the 5e games I've played, I usually don't feel like my character's life is on the line -- resurrection is cheap.
-That being said, there are a lot of situations in OSR games where you don't roll--what matters is that you narrated the right solution. In the game I'm running, there are no *mechanics* for finding or removing traps, it's all handled by narrative. If you spotted the trap, and you describe the right way to jam it/bypass it, it works. You have to listen to the DM's description and do problem-solving. You can't ignore the narrative and then coast on your stats.
-In my game, you don't get XP for killing monsters. You get 1 XP per gold piece you bring back to town. This shifts how you think about combat. You're trying to steal treasure from the dungeon, not "clear" the level. In 5e, if you bypass a combat encounter, you're usually cheating yourself out of XP. But if Gold Won = XP, then talking, avoiding, distracting, and pitting monsters against each other becomes a great strategy.
-OSR games do not usually favor "buildcrafting". In modern D&D, when the DM describes a challenge, players tend to scan their character sheet for the most appropriate skill or ability that has the best number next to it. This style kinda shortchanges player's creative problem solving.
-And that's because OSR challenges tend to not be about the game math ("How do I hit AC 25..?"). Sometimes the challenge is, like, "there's a tiny octopus in your belly and it's biting you!" It's not a problem you can solve using any of your skills or class features, you've gotta come up with a creative solution. You might have to repurpose tools, invent new ones, or think outside the box. That's where the magic happens.
-Most editions of D&D have clunky mechanics for encumberance, wandering monsters, and tracking time. In a lot of games, these things become accounting exercises, or roadblocks to the next story point, so it's no wonder that most D&D tables handwave them. But in the right context, they can be a very interesting and dynamic source of challenge. When you're planning a delve, you have to strike a balance between moving quickly, being prepared for any given situation, and having enough room to carry treasure out of the dungeon. I LOVE the moment where a character is weighing these things against each other -- "Hmm... if I drop my shield, I can carry 500 more gold out of the dungeon..."
-And finally, the oldschool D&D aesthetic lends itself to CLASSIC ROCK AND HEAVY METAL.There's a certain magic about playing D&D while wearing heavy metal t-shirts, drinking beer, and listening to Blu Oyster Cult. These days, I also prefer Sword & Sorcery to High Fantasy -- Conan the Barbarian, as opposted to LotR. Frank Frazetta. A Wizard spraypainted on the side of a van. For years, I've been playing D&D to fantasy movie soundtracks, orchestral music, trying to make it feel magical and epic epic epic... I have only recently discovered I kinda like Black Sabbath? And that gritty, 80s fantasy aesthetic can be very tasty.