Here are some differences that were baked into the rules, pre-WotC
1) Monsters were scary and invulnerable to basic attacks.
If you wanted to harm a werewolf, you had to have silvered weapons. You had to study its weaknesses. If you contracted lycanthropy, you had to perform a ritual to undo the effects. (Wolfsbane, full moon night, killing the werewolf that gave it to you.) [Today this is damage reduction - so it effectively just has double hit points. Lycanthropy is a simple saving throw or cured with a spell.] See also mummies, vampires, etc. The monsters actually resembled creatures of folklore and legend, not just stat blocks made to fit the game.
2) Spells were open to creative use.
You could use Command to issue any number of commands, instead of a pre-ordained list to hamper creativity and usefulness. You could cast Light on the eyes of a creature to temporarily blind it.
3) Spells were effective
Hold Person, Sleep, etc., lasted long enough to sway a combat. Now, they last for a round - if you're lucky enough to have a creature fail its save. This means that the only option available to most parties is to swat at an enemy until its HP are whittled away.
4) Poisons were scary
Now they're just HP damage, maybe giving you disadvantage for a round.
5) Passive Perception
Finding traps and spotting enemies required paying attention to the fiction, the DM's description. Interacting and asking questions. Now, you don't even have to roll a die. You can play on your phone, scroll through social media.
My statement isn't that post-3.0 D&D isn't a fun time. It's that maybe enough has changed that it's a different game from TSR-era D&D. Like as different as Savage Worlds is from Genesys.