Can a DB handle very large single documents?
Can the DB handle very many small-to-medium documents?
Maximum database size
Granularity of locks: per-document or smaller?
XQuery 1.0 support
XQuery Full-text search support
Update support
Schemas: required, optional, or unsupported?
Import formats
XSLT support (a la FileMaker)
And of course cost, freedom, and support