I last DM’d 1st edition around 2002, and last played 2e around 1989, but what I remember about why I preferred 1e:
- Half orc as a PHB race. Interesting character choices.
- Assassin as a PHB class.
- 1e Core Rules did not have classes I thought of as newfangled from Unearthed Arcana. I think Barbarian and Cavalier became PHB classes in 2e.
- 1e called Demons and Devils by their proper names. In general, 1e was more metal and more based in mythology/religion/history, whereas 2e was the start of self-referential D&D.
- Greyhawk instead of Forgotten Realms as the default setting
1e- more character classes. I mean, sure, you could play 1e classes in 2e, but 2e classes were usually weaker than a multiclass or anything in the PHB. I remember playing a Scout from Dragon and being way happier than I ever was as a Thief. Also there was just more things you could be in 1e. Unearthed Arcana is all like, oh, so you want to be a Halfling Druid/Thief? Sure, why not?
There's something about 1e's presentation of the game that makes it feel somehow more fantastic to me. Like I'm reading some tomes detailing an incredible, magical world. Too much of 2e is matter of fact, telling you all the secrets. Like, 1e would off-handedly mention something like "The Rain of Colorless Fire". 2e would turn that into a boxed set!
Better adventures. This is going to be somewhat controversial, but I don't care for most 2e adventures for some reason. I'd much rather play Beyond the Crystal Cave or Expedition to the Barrier Peaks than Swamplight or Tales of Enchantment!
2e- higher level limits for demihumans. Whole business was a mistake; should have just given humans a racial xp bonus. Way more options for characters- I couldn't live without my Complete Fighter's Handbook (and some of the other ones are ok too). Mythos Priests- sure, they got entirely out of hand, but it was so much cooler than being a generic Cleric. Tons of exciting settings! Lots of zany gonzo things.
You want to play a Wild Mage who is a living Wand of Wonder? Or a metal cube with robot limbs and tiny wings? Tame. Let's talk after you make a Tiefling using the tables in Complete Planeswalker's. Or an Alaghi or Fremlin out of The Complete Humanoid's Handbook. Or a Ranger who is turning into a living tree!
Not overall but two specific rules.
1. Multiclassing. 1E was more generous in regards to MC clerics and perhaps weapon specialization.
2. Magic resistance flat number eg 50% vs +/-5% per level under/over level 11.
I've never played 1e, but I've read through some of the differences, the two you've mentioned here I prefer the 2e multiclassing rules but much prefer the 1e MR rules and I'm pretty sure I brought it into me 2e games, though it was a long time ago.Not overall but two specific rules.
1. Multiclassing. 1E was more generous in regards to MC clerics and perhaps weapon specialization.
2. Magic resistance flat number eg 50% vs +/-5% per level under/over level 11.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.