One thing that the book did for me was put to bed any lingering notions I may have harbored about 4e being just a miniatures game. I mean, I didn't necessarily hold to that opinion, but I heard it expressed frequently on the internet. This book, at least, was primarily a setting book, and while it had some mechanics in it, it was mostly a book that could be used regardless of system. It is a book that I could use fairly easily in a 3.5 game, or for that matter, in any fantasy game using any system from FATE to Savage Worlds to HERO or whatever else you'd want to use.
It was chock full of D&Disms, though. In spite of the fact that many 4e detractors have complained about how the 4e implicit setting is erasing years of history and D&Diana, I found this material extremely "D&Dish." Most of my homebrew settings, even while cleaving faithfully to the concept of the D&D game (something that I frequently don't make a point of doing) are much more divergent from the creaky old Great Wheel cosmology than this is.
That said, some of the most iconic D&D settings, including those published by WotC for 3rd edition, didn't use the Great Wheel either (I'm thinking of Forgotten Realms and Eberron here) and the 4e implied setting isn't really any more divergent from the Great Wheel than Forgotten Realms "Tree of Worlds" or whatever it's called is, and it's probably a good deal less divergent than Eberron's orbiting planes cosmology.
The gist of the Plane Below is that it's a combination of what were a few completely different planes in the 3rd edition Manual of the Planes and before. Much of the Elemental Chaos is like Limbo, but it also manages to be all of the four iconic Elemental Planes as needed as well, and the Abyss is buried deep within the Elemental Chaos as both a part of that plane, and yet also a plane unto itself. The thinking here, according to the WotC designers, was to make some of these planes more useful and attractive as actual adventure settings, rather than theoretical places that you couldn't figure out what to do with in game. In this goal, I think they were successful.
In others, perhaps less so. Personally, I think they didn't got far enough, and the D&D outsiders is still full of a lot of overlap in concept. While they did take some efforts to distinguish the slaad, for example, from the demons, it was a bit too little. Why aren't the slaad just some group or tribe of demons? Because of lingering sacred cows, that's why. What's the difference between a demon lord and a primordial? Especially an obyrith demon lord? Nothing substantial. At least not that's spelled out here. Pretty much all of the primordials could be called new demon lords and nobody would know the difference.
That said, the actual ideas that this book had are pretty good. Many of them are older ideas that are just restated for the fourth edition, but some of them I didn't recognize. Granted, my D&D lore may not be up to snuff, but I think they went to some pains to actually create some new material, not just represent older ideas from planar adventures.
This book certainly didn't convert me to 4e, nor did it do anything with the cosmology that was so intriguing that I'd look at adopting it (except a handful of extremely discreet units of setting design that'd be fun to borrow from time to time) but I did find it an interesting read nonetheless. Not the equivalent to the excellent Fiendish Codex I: Hordes of the Abyss, but interesting nonetheless.
For my next RPG reading trick, I've got some Pathfinder material up my sleeve. I had picked up the Absalom sourcebook off my shelf, not with the intention of rereading it word for word necessarily, but to do some research for the short story competition I mentioned last time. That got me in the mood for more Pathfinder, and on a trip to my local gaming store, I ended up five books heavier, including Heart of the Jungle, Classic Horrors Revisited, Sargava: The Lost Colony, Faction Guide and City of Strangers. These books aren't very long, and I can often read them pretty quick. In fact, it's possible that I'll read them quick enough that they may not even end up on my What I'm Reading list at all before going straight to the reviews. We'll see.