I agree with a lot of what people here have already said. My favorite stuff from 4e:
-Clear rules: The game tells you upfront how things work, which allows you to spend less time with rulings and clarifications, that you can use to play the game.
-The updates: During 4e's life, we got a lot of updates to the system, that really improved it in the long run. Be it the monthly errata, that constantly fixed the bugs of the game, or the whole revamping the rules got in the later half of 4e life (Rules Compendium, Essentials, etc.). When 4e ended, the system was as perfected as much as it could be.
-Easy to DM: The clear rules allow DMs to actually enjoy the act of DMing. Encounter building was easy and straightforward. You don't have to spend hours and hours of preparations for a single session, monsters stats are filled with the precise information you need to ruled combat (no longer looking other books in search of this specific spell the monster may or may not use in the combat), which also mean that modification and creation of new NPCs and monsters was really easy...
The lore: Perhaps my favorite part, I really love the lore and concept of the Points of Light world. It's a world that makes sense in the context of D&D. The mythology and the gods make sense and is integrated into the world, so it affects it and makes sense.
The adaptability: 4e was really easy to houserule. Do you wanted to cast a 'Frostball' instead of a Firebal? Just reskin this power and be happy. It's actually encouraged in the books. You need an specific monster that don't exist in the rules? Just use this other monster, change some keywords, and it's done. You can port encounters from premade adventures into your campaign with little and easy modification, etc...