I've been doing a lot of looking on flickr, since there are a ton of images with a ton of cameras and a ton of technical levels - trying to figure out what that pop is for me.
I think there is a quality to MF that is different from 35mm. I am entitled to my own opinion, no matter how wrong it may be
I think that in looking at a lot of images there are certain rendering characteristics that I really grove on, and the lens rendering I seem drawn to is the Zeiss look. I find that 70-80% of the hasselblad images I look at are on the nose for me. Probably a bit higher with Contax 645 (but this may be a user issue, since the person buying a connate 645 is probably not a novice... and its way out of my price range anyway) A similar percentage of the RB67 images do it for me, and 50% of the Mamiya 645 images do it. I also really like the Fuji GW690 (I shoot 4x5 with a fujinon lens which I also like a lot)
I know that a lot of the end result is the nut behind the camera, rather than the body or lens, and maybe the film to a degree as well (Portra seems to have a quality I like in color)
I have one M645 lens that I use on my A7 (120 f/4 macro) and the reason I'm looking at the 645 is to use that lens, (because I'm a cheapskate).
I'd say probably if money were no object, A hasselblad would do me fine, but I live in a world of mortgages and a limited "hobby fund", so I'm looking at cheaper options.
Hope that helps...