Less heavy & shorter is very nice.
And what about the IQ and other possible improvements?
Shorter and lighter would be very nice, after a day walking about with the current 24-70mm on a gripped 5DmkII hanging off my neck I feel some aches and pains...maybe I just need to work out more
if by "hanging off my neck" you mean you're using a conventional neck strap for that kit, your neck and back would thank you greatly if you moved to a sling strap like Black Rapid or one of it's competitors. I can carry my gripped 7D/70-200 2.8 with an R-Strap for hours with no discomfort.
+1 My RS7 is easily one of the best purchases I've made. Get any R-Strap and weight won't be an issue anymore. If you still have troubles getting the camera up to your face, then you may need to work out a little
I carry my 100-400 on a Black Rapid 7. Very nice, much "lighter" and it doesn't bang into the tripod like it did on a regular neck strap.
EOS: 7D (2, 1 gripped), 50D gripped, 580EX II + CP-E4
EF: 500 f/4L IS, 100-400 L IS, 70-200 f/4L IS, 28-135 IS, 1.4x TC II, EF-S: 15-85 IS, 10-22
5D3, 17-40 L, 24-105 L, 24mm TS-E II. S100 p&s