We've been fighting to get clients we have thinking of upgrading or creating new apps to also choose PostgreSQL in the process.
Here I'll just itemize some of the obstacles we've run into in making the sale. All of these fall under the It is not Database X line item.
By database X I mean SQL Server, MySQL, and Oracle and for us in exactly that order. Our obstacle focus is probably a bit different from others since
we are consultants to mostly Windows shops or consultants to ISVs who have to sell their applications to U.S. government agencies or units of agencies.