A lot of these aren't postgres-specific. (null weirdness, index column order, etc.)

For example, how nulls work - especially how interact with indexes and unique constraints - is also non-intuitive in mysql.

If you have a user table with a non-nullable email column and a nullable username column, and a uniqueness constraint on something like (email, username), you'll be able to insert multiple identical emails with a null username into that table - because a null isn't equivalent to another null.