d20 UTH: you got your fluff in my crunch
Wizards got too vested in making 4e "their own", as in their own Intellectual Property. And while the original D&D cribbed a lot from Tolkien and I could see a desire for WotC to divorce themselves of that, their desire to have their own fluff/IP led them to integrate it into the core rules of 4e.
Part of this is evident in the addition of the new classes, the Warlock (which I actually like and was added to 3.5 via a splat-book) and the Warlord, the latter made solely for 4e.
The new races and changes to races were also mixing new fluff with the crunch. Tiefling, which appeared in earlier incarnations' planar materials, was promoted to a core race, and despite their +2 Charisma bonus were given huge horns and a tail. And then there's the Dragon-born, the racial equivalent of the Warlord there to say "look what we made".
Then there's the gods and clerics. Previous additions always assumed that the DM would want to pick a pantheon(s) for his world, and gave guidelines for picking sets of powers (Domains) that aligned with those gods. In 4e, the new D&D pantheon is the default with no guidelines, rules, or even suggestions (as far as I've seen so far) for using other pantheons. I've cut and pasted Powers tied to the various 4e gods to gods in my campaign's new pantheon, but I could see it being a very frustrating experience, especially for a less experienced DM.
That's not to say 4e is bad. I am still converting my campaign (but I'm not deleting my 3.5 campaign info... if it goes badly "it was all a dream.") But I wish they had done like Hero and GURPS and seperated the rules from campaign world specific info.