The 70-300 VR's focal length and 62mm diameter (the design size) restricts the f-stop. (For example, Sigma's 100-300 f/2.8 is a monster lens because it's a constant 2.8.) With added size not only comes added f-stoppage, but added price. This is already a $700 lens. You get the price any higher and it needs to be substantially better built and be classified as a pro lens. I wouldn't mind seeing a revamp of their 80-400 VR lens with AF-S and a more useable f-stop.
On that note - I would like to have seen a slightly lesser built 70-200 VR at a constant f/4 as well (a-la Canon). That would be useful for portraits & outdoor sports action... not that I need one, but it would be less expensive than the 2.8 version for people who want that focal length.