>> Underestimating the amount of work and the cost of failure in maintaining data integrity at the application level - for all but the most simplistic applications you will end up writing a ton of code that replicates what a RDBMS does, except slower, with more bugs, and less generality.

If I'm the first to explicate the underlying rule, can I name it after myself? Please? Pretty please with Lisp on it?

Doffing's Tenth Rule of Database Systems (with apologies to Phillip Greenspun):

"Any sufficiently complicated database management system contains an ad hoc, informally-specified, bug-ridden, slow implementation of half of a good RDBMS."