I've posted a long article and commentary on the new Nikon D4. If you're interested in the camera, you'll want to read that from start to finish (and I'll update it as I learn more).
If you want the short synopsis: nothing revolutionary this time, just a great deal of iteration. Indeed, at least three things I've long requested have been fixed/added, so either Nikon finally got around to discovering those problems themselves or they finally got the message from users.
Videographers will be impressed with the D4. There's little Nikon got wrong with the changes to the video, and the D4 now becomes the best of the bunch in terms of DSLR-based video, I think. Full manual control, uncompressed output (via HDMI), remote control and monitoring, plus audio monitoring are nice additions. What did they miss? Dual mic audio, mini-XLR, AVCHD 2, under/over cranking, multi-camera syncing.
In terms of still use, it's mostly tweaking. Bigger buffers (with the right cards), better autofocus, and improved metering are the main bits. But there are some hidden pluses, too, including IPTC data entry, a built-in HDR mode for JPEGs, silent photography (2mp), and more. The 15% resolution increase isn't going to excite many, though it is welcome, and just enough to produce visible differences.
One gotcha with the D4 is cost. No, not the US$6000 price tag, which is in line with inflation and currency conversion changes. You'll need new batteries. If you want the best remote capabilities, you'll need the new WT-5 and an iPhone/iPad. You'll want new, faster cards (both XQD and UDMA-7 CompactFlash). In short, you'll spend a lot of money updating the ecosystem around the camera.
Again, read my full article for the details.