According to Monash:
every sufficiently large enterprise needs to be cognizant of at least 7 kinds of database challenge.
via Database diversity revisited. I can relate to that for the Financial Industry (not having had much exposure to other industries).
I wonder if maybe there should be an 8. challenge, to provide a cost effective (relational) database service for the 60-80% (OLTP) databases that don’t have any special requirements, and where the traditional RDBMSs require way too much DBA overhead, and your typical Oracle or SQL Server license costs way too much. I’m hoping that the likes of NuoDB and Xeround (AFAIK we still can’t get them for our internal cloud, but there’s still hope), but also SQL Azure or database.com (external cloud) can fill that gap eventually.