I have avoided this thread for three days because of the "train wreck" direction it was headed, but now it seems folks have calmed down to the point where some meaningful information may get ultimately shared.
I have shot MF digital for a while, but have never owned any of the HR digital backs, so my comments are relegated to my most current cameras to compare, the Mamiya ZD and 1Ds3.
Not wanting to repeat much of what has already been said, here are a few of my own observations:
First is re the MF "look": Yes, the 80 f2.8 lens on MF has a totally different look than an 85/1.2 on FF. Not even the same ballpark; the 80 on biggest MF is about an effective 55 FoV, yet the f2.8 aperture renders more like f1.4 in FF terms. Total DoF may be deeper on MF, but it clearly falls off to a smooth OOF rendering faster. And as pointed out, the MF 80's wide open are extremely sharp compared to most FF 50's at f1.4. IMO it is a pointless exercise to get into cropped MF frame with an 80/2.8 compared to FF with 85 -- anybody with a MF camera is going to mount a 140/150 (~ a 95 - 105), not crop the 80. If you compare a 140/150 f3.5 wide open to an 85/1.2 wide open at comparable FoV, the 85/1.2 will show shallower DoF at the PoF, but the overall look of the two images is surprisingly similar. Sidebar: I shoot with a Hassy 110 f2 lens on my Mamiya via an adapter which is about equivalent to a 75 on FF. Compared to the Canon 85/1.2 wide open, the Hassy 110 is significantly sharper at the PoF with DoF falling off *much* faster with the 110...
Noise: Go to 200 on my ZD back and you will see what looks like a lot of noise in the shadows, yet when printed to the same final size as an equivalent 1Ds3 at 200, the MF print is notably smoother.
MF has better tonal range and better color.
FF has higher ISO options, is more portable, faster frame rates, faster image reviews, better AF, better AE and better AWB. It also has broader selection of focal lengths both wider and longer, more zooms and more special-purpose lenses.
Weight and overall handling ease of a body with a prime are essentially a wash if compared to a pro-level FF camera like the 1Ds3. Obviously mid-level FF cameras like the 5D are much lighter in weight.
Other points of consideration: In MF, you have either focal-plane shutter bodies or leaf-shutter lens bodies. A body with a focal plane shutter can generally be adapted to use other manufacturers lenses. For example, I can mount and shoot with older Mamiya 645 manual lenses, Hasselblad F or CF lenses, several older generation 6x7 system lenses and even large format lenses if attached to a bellows. I am limited to a 1/125th flash sync speed with this system, a disadvantage in studio applications. On leaf-shutter lenses, you are generally limited to the lenses specifically designed for the camera body, but can gain flash synch speeds up to 1/800th second. Horses for courses...
Cheers,