You would be surprised at the number of throwbacks to Mystara/Known World that were put in to 4e. Of all the settings of ol' TSR, Mystara fits with the least changes into the new cosmology, IMO.
What makes Mystara special to me is it's a setting that acknowledges and embraces the crazy things players want to do in a magical world. Become a god/immortal? Sure. Build a flying ship shaped like a winged lion? No problem. Rule a country? Play as a troll? Fight an army of 33-HD gargantuan ogres? You can do it all. On top of that most of the world is wilderness and you can play for years in a few hexes in Karameikos.
I love FR and am not putting it down in any way. To me FR says "your character can meet celebrities" while Mystara says "your character can be a celebrity."
Note: I have to force myself to type the word Mystara. For most of the time I've been playing it was just the good old "Known World."