That's funny, it was the cosmology as described in Wizards Presents: Worlds and Monsters, which I picked up at the book store before 4th edition was released, that led me back to D&D after missing 3rd edition. The particular parts of the cosmology that I liked were the Shadowfell and Feywild, which I thought were brilliant, although I prefer the 5th edition cosmology because I like the way the Outer Planes -- the Abyss in particular -- and the Ethereal plane are handled in 5th edition more...and I missed the Positive and Negative Planes. However, the incorporation of the Material Echoes (which I understand had appeared in earlier editions, although less systematically) was brilliant. Also, I think the best part of the art in 4th edition was that of landscapes and realms.