Just reading that the 16-35 f/4 is almost the same size as the older 17-35 f/2.8. so f/4 doesn't really give you a size, weight improvement, but then again, it could be the VR adding to the bulk of the weight.
Also shooting DX at the moment, and from a quality perspective, I don't really find too much on offer from the higher end FX cameras other than the outrageous high ISO quality and the better quality of lenses available on the wide end. A 14-24mm/16-35mm beats the quality of a 10/12-24 hands down at the extreme wide end.
When I can get FX at sub $2k prices, I'll bite, as I don't have any FX wide angle lenses at the moment. 12MP is also good enough for me, looking at how Canon has started pumping 18MP into their consumer range. Shooting a few hundred photos on RAW at 12MP eats up space really quickly, not to mention the processing power/memory I need to just run through the photos. The money I save on the camera will have to go to a computer upgrade.