What I liked about 4e:
- swarms, minion, standard, elite, & solo monsters: giving monsters depth of challenge at each level.
- monster roles
- easy to make killer encounters and not worry if the players could take it.
- powers in general (exploits, tricks, spells, prayer, etc.) and "basic" attacks (we are looking at adding them to our 5e game)
- AEDU structure and then the options to not use that structure too (Essentials line)
- spells by character level
- healing surges (which we expanded and called heroic surges - great concept, just need to be pushed further, we use these in our 5e game)
- skill challenges (I like the concept, but the explanation & execution needed work, we still use these)
- action economy (though I prefer some aspects of 5e & PF2e now)
- character paths (epic destinies, etc.)
What is AEDU? Don't even recall that. At the same time I don't need or like any kind prescribed structure. I did not like the Controller-Defender-Damage dealer structure at all.
Monsters weren't terrible, though eventually became weak as my players mastered the game.