I'm ready to move to the Nikon FX line. Is the D4 image more nuanced and lively than the D800?

It's not good to have black borders or have the aperture locked (although I'm at loss to see why you would change it during video).

However, I also don't see it as a major issue. You can change the aperture if you use a lens that has an aperture ring. The black borders also don't show up on the rear screen, if you're using that to frame. It's only an issue if you're recording/viewing using the HDMI out connection.

Note that plenty of DSLRs, including the D700, 6D, and nearly all entry level DSLRs except the K-30 don't feature 100% viewfinder coverage. It's really not that bad. And you won't even see this on the D600 unless you use the HDMI out connection.

Oh, and the OP never asked about video...

It's more apparent than any Nikon, except perhaps the D3 and D7000. No one is denying the issue. However, it's an issue that can be resolved (by cleaning the sensor). It doesn't affect anyone unless they're shooting at narrow apertures, and even then they're hard to spot unless there's no detail in the right area and you know where to look. I also believe that the issue goes away completely after using the camera for a while. I had dust spots after some initial use, but they haven't come back after a cleaning. And I've had plenty of shutter clicks and lens changes after that cleaning...

Really, there's a lot to criticize Nikon on if you want to do that, but the dust issue is just a really minor thing to pick on.

How about having the ISO button on the wrong side of the camera (and this goes for every Nikon DSLR)?