Personally, back before JSON became a standard thing - I made heavy use of HSTORE to clean up some insanely cludgey string packing BS we'd been doing - leveraging recursive CTEs using the UNION definition allowed us to move a fixed tier system into a self-referential one that saved us a lot of headaches as well.

Other folks have mentioned :: instead of CAST AS (which is really nice when you need to write it a bunch of times) but it's mostly the diverse built-in function set and constraint support that keeps me an enthusiastic Postgres supporter.