True. But the question is: Is it 'flavour' that is useful to someone still playing 3e?
Honestly, I think the answer to that is "yes".
I've been constantly running D&D games in my version of the World of Greyhawk for the past fifteen years or so (and there were earlier games as well). In that time, I've gone through 2E, 3E, 3.5E and now 4E. Before 4E, I used the Great Wheel cosmology to some extent. That is, I used the demiplane of Shadow and the Astral Plane, and most of the rest was pretty much ignored: my adventures were set mostly on Greyhawk.
The influence of the devils and demons could be seen on Greyhawk, and likewise that of the deities, but where they lived didn't matter so much.
The 4E cosmology and mythology was a revelation. The ancient war between the Primordials and the Gods - so reminiscent of the Greek mythology I grew up reading - really struck a chord. What has helped that is through the later books and especially the adventures, the Dawn War informs what is going on today. With Greyhawk, as it didn't have a real creation mythology (despite nods to past days by Erik Mona, such as in the
Whispering Cairn), incorporating material from the 4E creation myth into the campaign hasn't been that hard. Obviously, some things have been altered, but I have certainly been inspired by the 4E works.
It is made a lot easier, of course, by the fact that I am using Greyhawk, as so much of the mythology of 4E is derived from older forms of D&D (which Greyhawk pretty much exemplified). When it discusses Vecna, Acererak and Orcus, you're talking about three beings that have put their marks on Greyhawk before, and integrating them is a lot easier than in some other realms.
Whilst the 4E mythology has existed since its launch, it hasn't always been presented well: you get hints in the
Monster Manual, but there's definitely too much crunch over fluff in that book. Once you start getting to the later books, the balance shifts more towards the descriptive and evocative text, enough so that the
Monster Vault has very many interesting descriptions of monsters.
A recent discussion on
RPG Geek had one poster saying how he really liked the expanded descriptions of the 2E Monstrous Compendium. There's no doubt they're expanded - many, many words on a page (and few stats, because that's how AD&D worked). However, I find myself preferring the Monster Vault descriptions which focus on the interesting things about the creatures and how they can be used in adventures. If you haven't seen the
Monster Vault descriptions, go borrow a copy: it's pretty impressive.
"Don't try to tell me there's a soul that can be redeemed inside that monster. An orc's little better than a demon." - Caiphas, paladin of Pelor.
Cheers!