I picked up Demonomicon yesterday - since my group is preparing to invade the Abyss tomorrow, I figure it was worth hunting it down when the early release hit the local game store.
I don't have it on-hand, but did take a good look through it, so can try and answer some questions.
Demonomicron: Fluff content and how it differs from existing content, what percentage guesstimated?
Starts off with a pretty in-depth discussion of the origin of the Abyss and the demon lords. Basically, even before all the current existence, there was another realm of existence that fell to pure evil, and when it began to self-destruct, the last remaining beings of that world - the obyriths - pushed a shard of their world's essence into ours. Into the hands of Tharizdun.
Of course, its power drove him mad, and when they showed up to conquer our world, the twelve that came through battled him to a stand-still. In the aftermath, the shard began to corrupt the Chaos, the Abyss was formed, the first demon lords began to fight over the Shard - some of them corrupted primordials like Demogorgon, others obyriths like Dagon.
Anyway, lots of fall-out from this that ties into the Dawn War, the constant strife over who gets to be Prince of Demons, the origin of the Rod of Law / Rod of Seven Parts, and more.
Interesting stuff, and feels like a solid expansion on the 4E mythos. Some of it might override earlier references, though enough things have been shrouded in 'possibilities' rather than definites that I couldn't point to any one thing for sure. Except for Asmodeus, they tie his corruption into Tharizdun and the Shard, and I'm not sure how I feel about that.
The next chapter, as I recall, goes more into the nature of demons. Again, substantial amounts of fluff here, though I mostly skimmed through this section. Includes information on demon possession, a ritual to summon and bargain with demons, a random table for what happens if PCs go around eating the scraps of souls that can be found in the Abyss, some fun stuff.
Then we get into details on the Abyss itself and a focus on a number of Layers. I would have probably preferred a smaller focus on more Layers, but the ones they do present are well fleshed-out (sometimes literally), and they fill them with various locales, planar properties, denizens, and you can easily see various adventure hooks there. Also, throughout everything, there tends to be small references to things that could easily have entire adventures built around them in their own right.
The focus was on the Plain of a Thousand Portals (Layer 1), the Blood Rift (Layer 4), Azzagrat (Grazz't's realms, of Layers 45-47?), Abysm (Demorgon's realm, Layers 88 and 90), and maybe one or two others - the realm of the Demon Lord of Wrath, and I think one about some Abyssal city. There was also maybe half-a-dozen more Layers that got shorter write-ups. Finally, there was a page with a list of pretty much all the commonly-known Layers and any relevant rulers. Various names come up here that I don't think have commonly shown up elsewhere in 4E.
Somewhere around here we have a few short adventures presented. One involving going into Abysm, another one involving PCs being manipulated by Shemeska the Maurauder, which actually could serve as a starting book for a longer plot.
Finally, lots of demons, and stuff on demons. A half-dozen themes to flesh out followers of Grazz't, Baphomet, Yeenoghu, Oublivae, etc. Some powers you can give demons in place of Variable Resistance. A number of abyssal hazards, though I would have liked more, and more abyssal terrain in general.
And then a boatload of demons. Four or five demon lords statted out, and lots of enemies, mainly across Paragon and Epic tiers. Mostly creatures of the Abyss, but also some demonic creatures in the natural world like Wendigoes or Ixitxachitl, or demonic clockwork horrors. Some creatures have solid fluff with them, others are a bit less inspired, but overall I felt this adds a ton of options and new toys for a DM.
So... that's a general summary of what I remember. To actually answer your original question, I'd say the book has a hefty amount of fluff, and it felt pretty evenly matched between fluff and crunch - the first few chapters are packed with it, and there is a lot of good stuff among the monster entries. Especially new enemies like Oublivae and such.
