My expectation is that if Canon wants to improve the value of the optically already excellent 300L II, 400L II, 500L II or 600L II, it must be related to faster AF, lower weight and/or new video focused features.
Sony is pushing the envelope with their new 400mm f/2.8 which is using a linear AF motor, maybe focusing faster than Canon and Nikon's versions. The Sony lens design reduces the weight and the front-heavy imbalance of current super tele lenses.
Video photographers are not happy about the current placement of the power AF ring towards the front of the lens and the AF stop buttons - they want it at the rear end. Mounted on a tripod, such a placement would be easier to reach (worst on 600mm and 800mm btw) and the front placement of the controls introduces maximum shake when you operate them rather than if they were placed near the camera. I guess that Canon has placed the controls where they currently are assuming that the lenses were to be used handheld.
Sony is pushing the envelope with their new 400mm f/2.8 which is using a linear AF motor, maybe focusing faster than Canon and Nikon's versions. The Sony lens design reduces the weight and the front-heavy imbalance of current super tele lenses.
Video photographers are not happy about the current placement of the power AF ring towards the front of the lens and the AF stop buttons - they want it at the rear end. Mounted on a tripod, such a placement would be easier to reach (worst on 600mm and 800mm btw) and the front placement of the controls introduces maximum shake when you operate them rather than if they were placed near the camera. I guess that Canon has placed the controls where they currently are assuming that the lenses were to be used handheld.