As an example, notice that Wikipedia does not require people to send modified articles back to the Wikimedia Foundation, or to allow them to use the data as they see fit (this is clause D. 3. d. in your license). They just require people to contribute under a copyleft license, and they can thus incorporate derivative versions published elsewhere if they want. This is nice because it ensures that the Wikipedia content can be useful even if the Wikimedia Foundation disappears.
Anyway, awesome work, congrats!
One possibility is so that database can be created on the backs of the users, then the database owner can slam the door and turn it proprietary, like the CDDB did.
Their stated goals don't lean that way, but I don't see a clause that either permits or forbids the database owner this action. Lawyers will be required to figure that out. I suspect there is an implicit ability for the grantor to terminate the license, but that is what lawyers are for.
(I did notice that the termination effects reference clauses that don't exist.)