A: Whether the other side can see your slide yet!
Nobody gets this right. Vyew doesn't provide any way of telling what your viewers are actually seeing. WebEx doesn't either. VNC can't give you that information.
What I'm envisioning is a little window (tuck it in below the video feed!) that shows a miniaturized version of what's on the other guy's screen. If the window sizes differ this will give you some clue what they can and can't see. If the connection is slow this will tell you when their screen has updated.
(Or, shade the portions of the screen that the other side can't yet see!)
Obviously there are problems scaling this up to N viewers, but honestly this seems like a basic application of the E2E principle and would be a great improvement over trying to verify over the phone that they're looking at the right thing.