I think apart from a rotating zoom (to which people seem to have mixed feelings) the usability with the TCs as confirmed by the MTFs, a reliable and more powerful IS, along with the much improved IQ is a sufficiently good reason for the widespread welcome this lens is getting.
Seems like the only thing missing is the much improved IQ. Roger's initial, cursory tests over at the rental place indicate it's only around 4% sharper on average. Of course there are other IQ considerations like flare, CA, distortion, contrast, color reproduction, etc., but 4% better sharpness isn't exactly a slam dunk.
To be clear, I'm looking forward to getting this guy and I want to love the heck out of every aspect of it. But 4% pales in comparison to the 40% improvement of the 400 DO II over its predecessor.