Are you talking medium format digital?
I've just made the switch, I used to use a Canon 5D Mk3 as my main body and have for the past few weeks been using a Mamiya AFDii / Leaf Aptus system.
As you say tonal range is improved, the files are native 16 bit, so can be pushed and pulled a bit further. The dynamic range also seems improved, I shoot landscapes and am having to use fewer grads than previously.
Finally there is an unquantifiable element, a bit like the difference between listening to a CD and listening to vinyl, the CD is excellent quality but the vinyl is better to listen to. As a guide have a look at my flickr photostream,
http://www.flickr.com/photos/markmullen/ where everything before and including the canoeist on the second row is shot with Canon dSLRs (5D3, 1Ds2, 1Ds, 1D2 and 450D) and the first 7 images are shot on MFD. They have a very different feel to them, I was quite surprised last night when I viewed them in that order for the first time to see the difference.
You have to watch DoF, and it seems to show up any deficiency in your technique that you might have got away with on 35mm digital. The viewfinder is awesome to use, the camera is a real weighty thing, the lenses are good and sharp and the lack of AA filter means everything comes out good and crisp without needing tonnes of sharpening. I've just had some prints made for an exhibition I've got starting tomorrow (at Joe Cornish's gallery in Northallerton) and the detail shown from the MFD files when printed at 600mm x 400mm is superb, even right at the corners.
Best of all I can remove the back and use it on a technical camera if I want, I've got a Linhof Technikardan which has amazing amount of movements suitable for architectural stuff. You could even use the back on a large format camera, an Ebony or Wista for example. As a system it is very flexible.
Finally, and it is only a small thing, sensor cleaning is a joy, the sensor is right in front of you, you can see what you're doing, and you're not proddling about down in a mirror box.