For longtime MySQL users (like myself, ahem), I've found the biggest initial hurdle to be the different syntax the stock MySQL and Postgres clients use. When dropping into a Postgres environment I have to constantly look up what the Postgres commands are to do simple stuff like list all tables or describe a table's structure. It's not that the Postgres commands are particularly abstruse or anything, just that they're non-obvious if you're unfortunate enough to have spent 99% of your life marinating in MySQL.
Postgres is often closer to the SQL standard. For example escaping column names is specified by SQL-99 with double-quotes (") whereas MySQL by default refuses this and requires backticks (`, same as Oracle).
Compared to MySQL, the presence of schemas is a big win for me (same as 'user' in Oracle). Also, I can't move away from Postgres because it's the only DBMS which supports transactional DDL, which the ability to roll back an "add column" as part of a transaction.
Parent seems to be discussing the cli toolset, where many commands are composed of a backslash and one or two letters, whereas the mysql cli expects verbose commands, like SHOW TABLES and DESCRIBE tablename.
