It's different from the other New Testament writings probably mostly in its literary quality. It is the finest literary Greek in the New Testament, incredibly well-structured. In many ways it is written like a theological essay meant to convince it's hearers of a particular perspective concerning who Jesus Christ is and what he accomplished on the cross. So, similar in that it is occasional, an occasional document, a true letter in that regard; different, it's got more literary qualities. It's highly theological in its focus, very creative theologically as well.
Dr. Mark Strauss Professor of New Testament at Bethel Seminary in San Diego, CA