I like a lot...
Favorites probably are:
- Fey Wild
- The history of Bael Turaeth, Arkhosia and the Nerathi Empire.
- Fey Wild
- The Dawn War and the Primordials as the gods' antagonists
- Oh, and I kinda like the Fey Wild. To me it seems to be something that was missing from D&D all along. A place for the fey and a lot of traditional "folklore" about Faeries.
- Primal Spirits. At first it seemed kinda redundant, mixing fey and divine stuff in some way, but it turned out to be interesting. Primal Power has probably some of the best fluff in that regard, with all the Primal Spirit descriptions and powers linking back to them. That's cool stuff.
The Underdark books good, and it makes it a more interesting place to me than it ever was before. I also like the Undead "Physiology" concept described in Open Grave (The "trinity" of Soul, Animus and Bod. In my Arcana Evolved campaign, I did use a four-part setup - Spirit, Soul, Mind and Body. I still kinda like mine more, but the Open Grave part also made sense to me.)
I think in general, the fluff parts are weakest when it is the "repeatable" stuff - like for powers particularly. But the Points of Light setting and its elements are usually evocative and can serve as a good inspiration for a campaign.