Hi,
The answer is much simpler than you would guess.
On technical issues it's a big difference:
1. higher bitdepth, meaning smoother transistions
2. no AA filter, so razor sharp images out of the camera
3. Higher dynamic range
4. very clean iso 50-100
5. Real ISO25/50
6. With some systems higher sync speeds (up to 1/1000)
7. Larger sensor so better control over DOF
8. better glass, shooting wide open is sharp
9. better color due to better profiles/sensor/combination/software
10. much better tethered shooting solutions
In the end however it's all about workflow.
MP's are not important anymore, with 22MP you can do most work without a problem, maybe if you need to crop alot or are just loving to zoom in a 56-60MP back is great but in reality most people don't need it.
For fashion and landscapes it can be a big plus, due the higher MP count there is less chance of moire and for landscapes more detail is always nice.
Moire by the way is in most MF software removed in seconds, some people are naming this one of the disadvatages of MF but also here the rule is, try it with the original software like Leaf Capture for Leaf, Phocus for Hasselblad and Capture One for Phase one, all have moire filtering that work very well due to perfect coorperation with their own cameras.
Back on workflow.
I shoot with several MF systems and several DSLR's.
For the quick work, low light, action stuff the DSLR is wonderful, however when shooting in difficult situations and you need direct confirmation the tethered solutions of most MF cameras are a great tool.
I'm always shooting to a laptop for important work, when I use a DSLR I'm hitting the buffer sometimes slowing me down, with a MF camera (although you're shooting less frames per second) there often is NO buffer, in other words you can keep on shooting untill the HDD is full.
This is a BIG plus for doing fashion work.
Some people will say the handling of the camera is bad, or you need to shoot from a tripod, those people life in the past.
With the modern MF cameras like the Leaf AFi, Hasselblad, Mamiya/phase one systems you can shoot handheld and AF is slower than a DSLR but for most work more than adequate, and MF is no problem at all due to the very large and bright viewfinders, also being able to use a WLF is a big plus for portrait and fashion.
In the end it all boils down to what you need.
If you need to push the files MF will give you MUCH more play than any DSLR file, if you want to shoot, do some color correction and deliver a DSLR will work fine.
It's horses for courses.....
A DSLR is good in almost anything now a days.
A MF is PERFECT for some work.
For example.
When fighting the sun outside a MF can drop to ISO25 or ISO50 use a higher sync speed and still deliver a picture with a wonderful DOF but with an incredible ammount of light.
This was shot in the FULL sunlight with a flash overpowering the sun by 2-3 stops making it look like a studio shot, dropping to ISO25 did the tick here, I could even stay at the 1/125 that camera gives me.
You should decided which camera fits your needs and use that "tool" don't look at MP only and judge a system, I use both systems and love both and could not live without both.
Greetings,
Frank
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http://www.frankdoorhof.com
http://www.doorhof.nl/blog 
