I like that the enemies don't respawn so you can't just grind your way through the game. I prefer that you don't know what items and tomes you'll get, and have to adjust your build accordingly, rather than decide literally everything about your entire build/playthrough in the very beginning like in D2.
The only difference is that in D2 you can't actually save in a single mode. But you can start over in D1 with the same char any times you want and venture let's say in catacombs right away if you did unlock entrance before. Same way you can farm gold, buy any amount of books and learn your skills up to 15 etc.
That's a pretty fucking massive difference.
In D1 you need to explicitly restart the game with that character at which point you know you're no longer playing the game normally. OTOH in D2 you physically cannot avoid playing it this way. Whenever you have to take any sort of break, everything resets.
Then there is the fact that unless you're playing hardcore, you don't really die in D2, you just respawn.
This lack of persistency makes the gameplay in both fundamentally different - finding specific things in D1 is a matter of if, in D2 a matter of when.
And finally, D1 capitalizes on that by drawing deeper from its roguelike inspirations - quests are random, enemy roster is random. Even on subsequent playthroughs you feel like you're doing something new, while in D2 you're just going through the motions.