I really love 4e because of the PoL setting. I like its associated lore, such as the Dawn War, the World Axis cosmology (for reasons already stated here by many), and the lore behind the Nentir Vale's world. I like that it's a dark setting, but also a Big Damn Heroes setting, making the players the protagonists (instead of the settings' NPCs). I love the fact that while it's a "traditional" setting, is not as humanocentric as the rest of them, leaving room for the non-human and non-"Tolkienian" races to shine. And I also like that is a "blank setting", allowing you to customize it as you see fit.
I guess is like Greyhawk in that regard, but more fantastic-oriented (instead of real history simulator with Tolkien's stuff). I like that.