At its simplest, we are still playing 4E, because it is thus far the edition that best suits the play style of both me as a DM, and my players (though not all of them would admit it).
It's nice to see my players trying out class and race combinations that never would have considered before (my wife playing a Wizard or a Cleric... HAH!) and enjoying it.
It's nice to see a combat that slowly evolves from a shifting stalemate, to a near defeat, to a rallying comeback, to a total victory for the characters. To have it happen over the course of several rounds, and because the characters came up with good tactics, working together as a group to succeed.
That's a start...
But since I saw this and the legion of THIS responses, I got to ask why this? The enjoyment of designing an adventure to me comes from the story and personalities I create. The mechanics of a system are totally irrelevant to that. What does 4e give you that makes it fun to create an adventure for? Honest question, not trying to be snarky or a threadcrapper.
For me, at least, if I have to spend less time fiddling with monster and traps statistics, I have that much more time to work with the story and the personalities that you mention.
In other words, I can devote more thought to the plot of the adventure, if I don't have to work so hard on the mechanics and system.