Why is column default value not portable through different RDBMS? Some ORMs (doctrine) do not support default values in database schema definition, because they states it is not portable.
But SQL92 supports the DEFAULT value clause, and all the databases supported by the ORM (MySQL, PostgreSQL, SQL Server, Oracle) support it too.

What are the pitfalls of the DEFAULT clause in different vendors/versions of RDBMS?
An ORM that is intended to be truly storage-layer agnostic will generally assume the absolute minimum from the data layer (tables, rows, columns of standard types, indexes, PKs, FKs, and that is about it) and will implement everything else like defaults and more complex constraints themselves.

While every relational database I could mention (mssql, oracle, prostgres, sqlite, mysql, ...) supports default values as defined in SQL-92, things vary a lot more once you consider using document oriented DBs and other less traditional arrangements for your storage layer (they might not support them and if they do the operating semantics may not be quite the same).