One feature that stuck out to me was the personal nature of the letter. Paul talks of his personal struggles and the possibility of facing death (Phil 1:21-26):
(21) For to me to live is Christ, and to die is gain. (22) But if I live in the flesh, this is the fruit of my labour: yet what I shall choose I wot not. (23) For I am in a strait betwixt two, having a desire to depart, and to be with Christ; which is far better: (24) Nevertheless to abide in the flesh is more needful for you. (25) And having this confidence, I know that I shall abide and continue with you all for your furtherance and joy of faith; (26) That your rejoicing may be more abundant in Jesus Christ for me by my coming to you again.