Prakriti
Hi, I'm a Mindflayer, but don't let that worry you
Ability score modifiers being streamlined. Honestly, this is probably the biggest change between AD&D and 5th Edition. A 16 in 5E is always +3. +3 to what, you ask? To everything, no matter the ability. Whereas in AD&D, it might be a +2 to hit, +3 to damage, 35% chance to bend bars, +1 spell slot, +5 known spells, +10% to pick pockets, +15% to open locks, etc. It was all too much to remember.
And I really disliked the fiddly AC's in AD&D. Attacked from behind? You lose your Dex bonus, and the attacker gets +4 bonus to hit. And you don't benefit from your shield, so that's +1 or +2 or +3 or +4 depending on the size of your shield. And if you're a halfling, then you get an AC bonus against large creatures. You almost had to keep track of your AC for 6-7 different scenarios.
And I really disliked the fiddly AC's in AD&D. Attacked from behind? You lose your Dex bonus, and the attacker gets +4 bonus to hit. And you don't benefit from your shield, so that's +1 or +2 or +3 or +4 depending on the size of your shield. And if you're a halfling, then you get an AC bonus against large creatures. You almost had to keep track of your AC for 6-7 different scenarios.