FR et al have well developed histories. In DF you're creating history. Anyway, for me that's the fundamental difference (and I should mention that I like the three worlds you mention and am currently running two FR campaigns). Blackshirt5 covers the world in more detail here: http://www.enworld.org/forums/showthread.php?t=73959
Edited to fix spelling error in Blackshirt5's name.
I'm DM'ing a FR underdark campaign, which will soon end. After that I may DM a short rokugan campaign, and then Midnight (and I have a nice ending in store for the players, but since one of them is here from time to time, I won't reveal that). I also plan to make a nother short campaign (Midnight, too) in another group.
In addition, I'll soon play in an evil ravenloft campaign (Vampire versus Lich - going to be fun).