The lore was the best D&D has ever had IMO.
Non armor defenses added some subtle dynamics like monks mostly attacking NADs, etc.
Essentials classes added a lot more freedom to the game overall, and brought some of my friends back in.
The Assassin. Technically underpowered? Sure. Still fun as hell tho.
Warlock Pacts, Themes, Paragon Paths, Epic Destinies, all had tons and tons of flavor to hinge a character on.
Rituals…I miss rituals. 5e spells with the ritual tag are not it. Better than nothing, but still very much lacking.
While I don’t miss all the stacking attack and damage bonuses, I do miss being able to stack class feature, power, magic item, and feat, to turn a 3 square teleport into a 7 square teleport, or turn a 2 square forced movement into 5 squares, etc.
Monsters/encounter design was top notch. I’ve yet to see any crunchy TTRPG that does it as well as 4e D&D.
Related to the above, skill challenges could be integrated seamlessly into combat encounters, because they both worked on a turn by turn basis, with some sort of action each turn. Factor in hazards, and the possibilities are truly magnificent.
Daily powers and the “until the end of the encounter” concept allowed for very interesting stuff like Bard songs that helped the team for the entire fight, but weren’t spammable. (I’d still kinda prefer it if you can use X daily powers per day, and if you want to use the same one all day that’s fine)
Spellcasters having implements, and the ability to specialize in a type of implement, and feats like the one that let you dual wield implements and gain extra damage or whatever. Eventually they added Tome implements and I was very happy. Let me use my spellbook to cast my spells!
Clear main stat for every PC, with attacks by main stat.
Magic items could be used as a “magic mart” deal, be up to the DM, not be featured at all with inherent bonuses, or some combination or mid-point, and you could have the same items just level up and stay mathematically relevant by upgrading to the level XYZ version of the same item.
Alchemy. Like rituals, it was a solid system and a robust set of options.
The eventual state of the ranger was very good. I disliked the mundane ranger, but once I could grab primal utility spells, and some of the later ranger powers were a little less strongly bound to being martial in effects, I loved it. The still dropped the ball on the beast subclass.
Rogues and warlocks kicked ass all over.
Swordmage, Warlord, Warden, Seeker, Avengers, are all awesome additions.
I liked wizards and fighters and paladins for the first time.
The races all had very solid, interesting, compelling, lore, and aesthetics.