This is not a review, there are tons of reviews out there. I just like to share the impressions I got after “upgrading” from D80/90 to this camera.
Size and weight. It doesn’t feel that big. The weight is relative, with a 70-200/2.8 it actually handles better than a smaller body with such a lens. With the 24-85 VR it is noticebly bulkier than a D90 with a 35/1.8, but barely different from a D90 with a 17-50 Tamron.
Resolution. The angular resolution is at the level of a D7000, so keeping the speeds at the level where I was comfortable with worked out. Using Auto ISO the D800 adjust the speed to 1/focal length. Due to the good grip and the well designed shutter button I get enough 100% sharp pictures out of it (you can still tune this in the settings)
Dynamic range: Mindblowing. In Lr4 pull down Highlight by 100 and pull up shadows by 100 - no visible degradation, not color shifts.
High ISO: Auto ISO works really well, but shooting at 6400 is still OK. At pixel level the results are like a D90 at 1600ISO, scaled down it equates a D90 at ~ 1000ISO. For normal usage (web, small prints) perfectly OK.
AF: I never used Auto selection, but the D800 changed that. When shooting a face it locks on the nearest eye. Perhaps in 98% of the case it makes the same decision I made only faster. I can concentrate more on the composition and let the camera figure out the rest.
Moiré: Yes, it exists. But the Moiré brush gets rid of it. I customized this brush by adding +12 saturation and after a quick swipe with this tool it is gone. It is important that you know where to find it, otherwise you might miss it.
Exposure: Very different from the D90. Spot metering is spot metering, you have to be very careful to pick the right spot. Matrix metering looks a bit underexposed sometimes, but it is not a flaw: It tries to preserve highlights and thanks to the excellent noise characteristics you can pull up shadows to almost no end.
File-size: I always shoot RAW, current setting 14-bit lossless compression. This gives you ~90 pictures on a 8GB card. I will experiment a bit with these settings, perhaps using uncompressed files to speed up post-processing.
Still, it is not a beginner camera, you need a good shooting technique, good glass and a solid understanding of the principles of photography. The camera is so good, it mercilessly shows you your mistakes. It will make you a better photographer.