I've just upgraded to the new 400mm f/2.8E FL from the old 600mm VR. It's purely a weight thing; although I loved it, the old 600mm is very big, heavy and awkward to carry around. I damaged the cartilage in my knee a while back, so weight is a big issue. I borrowed the 400mm for a trial, and it's become my current baby. Aside from the weight, I was also hugely impressed with how versatile it is, working so well with teleconverters, so there is no real loss switching to a lens with less native focal length. I also love the 18-35mm G – it's a brilliant little lens. OK, it's not pro grade, but the quality is spot-on at f/8, and because it's a wideangle, stopping it down isn't an issue. It's so small and compact that it's very easy to put in your bag. I've also got the 200-400mm f/4 VR – the older one – the 50mm f/1.4 and the two main teleconverters.
I recently sold my 70-200mm, and before that the old 28-70mm AFS, as I'm in the middle of an equipment transition (the 200-400mm will go next). As I've developed my style I've realised there were focal lengths I simply wasn't using. Optics are getting better all the time; newer versions are ever better, and tend to be lighter, too. So my kit list is getting smaller as I mature into what I'm doing. I'm no longer so worried about what I've got – it's more about how I'm using it. It's amazing how versatile even one lens can be. That said, although it's quite a niche optic, I've always had a soft spot for the 200mm f/2, and I've been hugely impressed with the recent 300mm f/4E PF.