Until I administered RDBSes from other vendors Oracle oft repeated "readers don't block writers, writers don't block readers' meant little to me. I really didn't know much about handling locking problems in 8 years as an Oracle DBA. 2 years of Informix and 3 or SQL Server and trust me I know lots more about locking.
So I would say, in addition to the comments about support and non-RDBMS features, add locking behaviour.
