Last month I visited India for EnterpriseDB and talked to many government and corporate groups. We don't have many independent Postgres community members in India, so I was surprised by the serious interest in Postgres and the number of large deployments under consideration.
The issue of porting applications to Postgres was discussed. Normally, when I think of porting to Postgres, I think of company-created applications that just require a proper Postgres interface library and simple SQL query modifications — and our community maintains porting guides for many databases, e.g. Oracle, MySQL, MSSQL, other. In addition, most open source application languages are rapidly moving to standardize on Postgres, so I thought most of the big hurdles for migration were solved.
However, there is one area that still has significant barriers for Postgres migration, and that is middleware, particularly proprietary middleware. If your application is written to use a middleware API, and that middleware doesn't support Postgres, there isn't much possibility of porting to Postgres unless you change your middleware software, and changing middleware software involves not just modifying queries, but rearchitecting the entire application to use a different middleware API.