Most of the stuff that isn't in a power, adventure, or monster description is
pretty kick-butt stuff.
I would have
Underdark's babies, if it asked me.
The idea of the formation of the Abyss is quite nice.
The Dawn War has a cool moral ambiguity.
I'm liking the cosmology better, as a whole. And I'm a big Planescape nerd, so you know there's something there.
Some of the classes are a little "oh, we needed a random justification for some mechanical trick we thought up!" (Seeker! Battlemind!

). The Far Realm/Psionics thing leaves me a little dry, but I do find it a good match. I just like my Far Realm a bit more Lovecraftian: it doesn't care about the cosmology. It behaves according to its own alien rules, and sometimes that sucks for the mortal things in its way, because they will be crushed. I'm not a fan of my Far Realm being actively invasive or anything. But the core "madness vs. stability" thing is clever, and very nice.
I even like the fluff behind some of the narmingly named things, like Shardminds. Reading their race, I found them kind of interesting, and I liked the "mental construct" thing differentiating them from the physical Warforged. Their name makes me want to stab things, but their fluff calms me down.
KILLOREN (not Wilden) don't suit me quite as well, but they do seem all "We invented a reason for this race we wanted to put in."
It can be a bit of a mixed bag, and unfortunately the dumb stuff stands out very prominently, but by an large, it's good stuff.
I think I have some misgivings about its flexibility, in light of that, but that's a trade-off. You can't have both significant fluff AND high flexibility. I'm kind of OK with the thrust of it being "interesting fluff." That gives me a reason to use the mechanics, and part of the fun I have as a DM is filing the serial numbers off of things and using them in new ways, which isn't as fun with generic things.