I currently use a Hassleblad H3Dii-31 and a Canon 7D, shooting for my own website which I'd characterise as "fetish fashion" (http://www.restrainedelegance.com
if anyone is interested - not safe for work...)
I use the Hasselblad wherever possible because of its clarity, colour rendition and sharpness. I like the big bright optical viewfinder, the delay on the mirror flip (which makes a perceptible difference in critical sharpness even with a very short delay), the fact that the camera compensates for focus shift at different apertures, and the leaf shutters which further reduce vibration and allow me to work with short shutter speeds with studio flash.The lack of optical low pass filter is a mixed blessing- increases perceived sharpness and micro-contrast, but is the devil's own job to shoot a model wearing stockings and suspenders.
Nothing stops the 35mm dSLR crowd from matching the salient points here to achieve similar levels of sharpness and perceived image quality.
However, I don't believe the dSLRs are there yet. My technique has probably got a bit sloppy but I'm spoilt by not having to worry too much about camera shake. When I have to fall back to the Canon and shoot at 1/160th of a second, with OLPF, and lenses which don't match the quality of the Hasselblad ones, I certainly notice the difference big-time at 100% zoom (let alone the lack of megapixels).
I don't shoot tethered and I dislike live view as a method of shooting. If I was a fan, I'd probably be trialling using my RED Scarlet for stills as well as video.
Until electronic viewfinders have resolution similar to the Mk I eyeball plus a Hassy lens and viewfinder, I want to stick to fast, unencumbered shooting with direct zero-lag vision of what's going on to pick the moment to press the shutter release.
The key improvement that would sell me on an MF upgrade is light sensitivity. The main reason I fall back to the Canon is available light shooting. Even though the H3Dii-31 has microlenses I find it hugely light-hungry, shooting ETTR I rate it somewhere around ISO 64. Indirect lighting indoors? Forget it.
There's always room to improve autofocus. I don't need more points- I have the Canon set to only use the centre one to focus and recompose anyway. But faster performance, especially in low light, would be welcome, and every little bit of accuracy helps - Hassy's TrueFocus sounds worthwhile. As various web reviewers are discovering, there's more to getting an accurately-focussed sharp 30+ megapixel image than slapping a finer grained sensor in a Nikon body- you need to pay attention to all these little physical niggles like focus shift, shift on recompose, etc.
A base ISO 400-800 sensor at 645 size with 40-ish megapixels and all the precision of MF? Sold.