Primary Keys and XML Indices: You can't have one without the other.
More of a note to myself: You can't create a XML Path Index without a primary XML Index, and you can't create a primary XML index without a primary key on the table. You also can't alter the columns in a Primary Key if you have an dependent XML index on the same table.
Of course, you can have a primary key without any XML indices, but that messes up my little mnenomic... :(