I think that the World Wide Telescope is a great product, much better than google earth.
There are a number of reasons, but I believe the major one was the level of collaboration and the true scientific nature of the product.
Overall, there needs to be a way to delay the tours, or have them load in the background. Background loading is a technology used by some web based television services that are integrated with microsoft media center.
My point is that you should be able to download a number of tours completely.
In addition, there needs to be a way to incorporate amateur images of common objects. That was one of the goals of google video which never really caught on but youtube did.
I don't think initially the product will scale like you tube. But if the technology is powerful enough, I would imagine the same structure can be used for anatomy to histology, or any scientific structure which combines knowledge with images.
Gary