* Integer columns with null constraints will silently translate nulls to 0s, rather than throwing an error (as constraints in all other contexts do). Imagine a database which, on violation of a foreign key constraint, just silently picked the first row in the target table - it's just insane.
* Transactions can't roll back schema changes, so if you have a Rails migration which fails on the application of a unique index in the midst of other operations, too bad. Your DB is stuck in the no-mans-land between migrations.
* Constraints are anemic relative to postgres: no integer range or string length constraints, no whitelist, blacklist, regex match, &c. &c.
There are certainly others I've run into, but the above indicate to me that the Postgres developers are working with/delivering a better code-base.