1. Nentir Vale. I've said elsewhere that I believe that it's the best official setting, and I meant it. I would happily play there until the end of my days, and I'm saying that as a huge DragonLance and Birthright fan.
2. Action economy was a huge improvement. As 2e showed, the attack action is an abstraction, including feints, parries, bad thrusts, and so forth. The hit that lands isn't the only attack, just the one that actually got through all the defenses. 4e's single attack action embraces this.
3. Weaving of narrative into the mechanics was really good in 4e.
4. Essentials was inspired. Love the thief, the hunter, the skald. Those books oozed flavor!
5. Multiclassing was both balanced and maintained the fiction that you were X but had Y training and experience.
6. Introduction of the Raven Queen and the Feywild. Still my preferred edition for the Feywild, and the Raven Queen was great.
7. The default mythology was awesome. Yes, it differed from the Great Wheel, but if it only applied to PoLand, I didn't mind. In fact, in my head canon, it's the original canon, and PoLand is the First World. It's only after a great cataclysm that the Great Wheel was fixed and the old lore as we know it.
8. Defenses were great!
9. Rituals were long overdue. Was a fantastic change that should have stuck IMO.
10. Skill Challenges showed a lot of promise.
Edit: I did love 4e. I actually quit posting here ten or so years ago for a long time because there was a lot of negativity towards it, though the mods did do a good job to squash anything that went too far. Still, it was hard to have conversations about it online, given the antipathy towards it at the time.