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
For me the internal zoom or IS doesnt matter so much. Aesthetically I don't like the look of the external zoom, but I've never (touch wood) had any issues with dust or moisture getting in over it's 4 years of use, I can see why taking it to Burning man would be scary though.
I appreciate the IS on my 100mm macro and when I am shooting people doing speeches in low light I sometimes wish I had the 70-200mm f2.8L IS II instead of the non IS version, but for my 24-70mm I've never ever felt the need for IS, that's just me though, I know a lot of people would like it.
The IQ improvements are what I am more intereted in hearing about, but I guess we won't know anything for sure until the lens is released and has been tested inthe field.