Postgres is highly extensible. I wrote compatible types for my mssql emulator to avoid exactly that problem (being Microsoft, they have their own ideas of how types should behave, and also I could make the underlying data format compatible with the wire format).
I'd be doing a complete parse and rewrite of the language though, so any of the language and type differences would be easily solvable. Some of the other differences your links highlight around foreign key and atomicity behaviour would be harder.
Actually, reading this week's LWN reminded me that PostgreSQL (up until the just-went-alpha 9.5) had no equivalent to the MySQL INSERT .... ON DUPLICATE KEY UPDATE syntax - what's known as UPSERT in postgresql circles.
Yeah I seem to remember MSSQL having a similar thing that I had to fake. James