Queries: reportedly, Adam Bosworth said at the XML conference...
"I don't know how we're ever going to truly optimize these queries."
Before we worry about that, though, we should consider how the average user is going to form those queries. The last twenty years have made great strides in forming SQL queries for numerical data analysis. The models underlying those queries fortunately support both the relatively non-technical users who wish to form them as well as the highly technical adminstrators who wish to optimize them.
What kinds of queries do these users wish to form? What are we trying to optimize? What relationships will these have with current typical business analysis?
Will we wait twenty years for the evolution to settle into a widely used winner?