Good point, and I think Dropbox can remain simple by default. It could then support an additional "advanced sharing" feature. This would allow exactly two extra

Good point, and I think Dropbox can remain simple by default. It could then support an additional "advanced sharing" feature. This would allow exactly two extra flags per shared folder, per user: (1) share read-only, and (2) share non-transitively. This should allow sufficient granularity for most access-control use cases out there. Anyone who doesn't understand what this means wouldn't use the "advanced" sharing features, and many more small businesses would be able to start using the product.