PostgreSQL Gotchas:

“PostgreSQL is a fully-featured, robust open-source database with strong SQL standards compliancy. As with all RDBMS products it has its odd little idiosyncracies, which while documented, are not obvious, counter-intuitive or just head-scratchingly odd.”

I’ve already run into some of the problems described there - “unquoted object names fold to lower case", and “COUNT(*) very slow".