Treasure packages. Divorcing rewards from killing things is a good thing.
Healing surges (+ Second Wind). Remove the cleric/healing dependancy. Have healing scale to the character.
Power Recharge simularity / Adventuring Day length / class balance. Because everyone (pre-Essentials) had roughly the same number of daily, encounter, and at-will powers, shorter or longer adventuring days did not throw off the balance between classes
Utility powers that weren't magic. Give equal utility to not casters.
Push for interesting combats. This was a combo of many monsters having more interesting mechanics, plus the push for terrain and hazards.
Skill Challenges. Even though the mechanics had some issues, the concept of moving away from individual task resolution was a huge step forward.
Supported Heroic Fantasy tropes. Everyone was that style of fantasy superheroic from the genre, instead of the split of "magic can do everything, but you can only jump 12 feet."
Rituals. Well, kinda. I loved the concept of them. However I didn't like that magic item advancement is an expected part of character advancement, and rituals used the same feeder mechanism of gold.
Residuum. Just wonderfully useful to keep the game going as a game, esp. with the gear-as-part-of-expected-character-advancement.
Keywords. Treating the rules as rules and making it easy to have repeated bits because they were defined. This isn't a knock of 5e's "ruling not rules", it's a statement that having common, repeatable, easily defined keywords makes it easier to work off them. Like Bloodied.
Unified Attacker makes the rolls. Doesn't matter if it's an attack or a save.
Easy durations. Things like "until the end of encounter" or "save ends" removed the need for tracking duration.