The 100-400 is, in a way, my standard lens, always on a 1Ds MkIII. It is something very different from what a 200-400 could be. Backed up by a 16-35 on a 5D MkII, it covers the 99% of situations, with great results in terms of IQ and image composition. I used my 24-105 may be three times in two years. Obviously is quite heavy, but going around with the above kit let me free of a backpack and I'm always ready to shoot.
Push-pull is ok for me, IS is not stellar, but good enough since 100-400 natural environment is usually well enlightened. Weather sealing should be welcome, but with very little attention you can survive without it. Last month I went for an hiking in a very rainy day. I found water right behind the 1Ds eyepiece cup, but keeping the resting lens inside my water proof jacket, was enough to protect the lens.
The only issue, for the kind of use I make of it, is weight. A 1D type body+ the present 100-400 is the maximum weight that I can afford at my neck for six-seven hours. By the way, I think that something can be done for that, also in case of improving optical quality by more glass. My 100 macro L plastic body is more solid than a metallic one and very light. I don't think that modern materials can't stand the heavier glass assembly of a tele zoom. A modern high quality plastic big white could be very fancy...