A few nice surprises--a four-page writeup of Sigil, the home city of Planescape. That was neat, it's only gotten a paragraph or two in recent supplements. A major fact error (that in the previous edition, no-one knew where Sigil was located, which wasn't true--it was demonstrably at the center of the universe).
Secondly, in the list of vehicles that one can use to traipse around the planes, is a class of vehicle called "Spelljammers" :) Neat!
A third interesting bit is that the Shadowfell strongly mimics early Ravenloft, with discrete realms that dark powers have complete control over, but are imprisoned within. Interesting. Particularly since Setting Book Ravenloft is pretty early in the release schedule, though presumably after Eberron and DRagonlance.
A lot of old gods from previous editions get mentioned, and the abyss looks like a fun, fun place to adventure in. I do hope for a follow-up with more details.
One thing I miss is the orderly layout of the elemental planes, which really hasn't existed for a good long time (the original Monster Manual had an obsessive-compulsively complex planar layout, which I lurved). 4.0 gets pretty far away from the "Wheel" structure. Having, say, the Elemental Plane of Water, the Quasi-elemental plane of Minerals, etc, as discrete adventuring environments was kind of neat, but the new arrangement has its own charms.
Interesting note--the paradises and hells have shrunk dramatically. The Nine Mountains of Paradise or whatever are a mid-sized state, about 700 miles long and a few hundred miles across. Pandemonium is only a good 30 miles wide. Weird decision. Even the Nine Hells are finite, though they're an earth-sized planet, so not such a big deal...there are almost no truly infinite realms. The Abyss is finite, but v-e-r-y large. Presumably, the Shadowfell and Feywild are only about as large as the earth is. The only really infinite spaces are the Prime Material Plane, the Elemental Chaos, and the Astral...everything
So, generally, one of the best books about the planes that D&D has ever released, very colorful. Maaybe a little light on crunchy mechanics, but not intolerably so.