I would like primarily to comment on two aspects of the letter. The first is its great precision. This precision is manifested in the careful explanation of the significance of the remission of the excommunciations of the four bishops of the Society of St Pius X, and the careful explanation of the status - or rather, the lack of status - of that society in the Church. In the paragraph which describes the future of the Pontifical Commission Ecclesia Dei as part of the Congregation for the Doctrine of the Faith, the collegial nature of the activity of that Congregation, and by implication, of the Pontifical Commission, is also carefully explained. The last sentences of this paragraph very beautifully offer a vision of the continuity between the teaching of Vatican II and the whole life of the Church over the centuries; they are addressed towards the leaders of the Society of St Pius X, and towards those who advocate Vatican II over and against the earlier tradition of the Church (cf my contention that Summorum Pontificum and its accompanying letter have a two fold glance, answers A2 and A4 at this post). Both the wording, and the tone communicated by that wording, are crucial to the future of dialogue with the Society of St Pius X.
The second thing I would like to comment on is the understanding of the Petrine ministry expressed in the letter. I would like to identify four steps in the way Pope Benedict presents this.
1. "... the overriding priority is to made God present in this world and to show men and women the way to God". This is a particular concern of the Successor of Peter, who has received the mandate to "strengthen your brothers", a mandate expressed by St Paul in terms of being "prepared to make a defence to anyone who calls you to account for the hope that is in you". If you read what I posted here about Pope Benedict's understanding of ecumenism as expressed during his meeting with leaders of other Christian denominations in Cologne, you will see the importance I think this has as a starting point.
2. "Leading men and women to God, to the God who speaks in the Bible: this is the supreme and fundamental priority of the Church and the Successor of Peter at the present time. A logical consequence of this is that we must have at heart the unity of all believers. Their disunity ... calls into question the credibility of their talk of God". And what follows is a key way of understanding the notion of ecumenism: "Hence the effort to promote a common witness by Christians to their faith - ecumenism - is part of the supreme priority". Again, one can see the parallels to the talk that Pope Benedict gave in Cologne; but here it is addressed, to an extent towards the Society of St Pius X, and towards the Catholic Church, instead of towards other Christians. "..to promote a common witness by Christians to their faith.." is interesting as a statement of the purpose of ecumenical activity, and perhaps worthy of further exploration.
3. "Added to this is the need for all those who believe in God to join in seeking peace, to attempt to draw closer to one another". This Pope Benedict identifies as the mission for inter-religious dialogue.
4. And, finally, this leads to the social dimension of Christian faith, the life of charity and care for others. Pope Benedict here refers to his encyclical Deus Caritas Est.
What I find interesting about this is the understanding of the mission of the Successor of St Peter that it expresses - and that Pope Benedict XVI does not just talk about but lives out as well. There is a clear connection between the mission for unity and the mission of safeguarding the doctrine of the Church, so it is not the case that unity trumps doctrine.
The Focolare Movement would, I think, recognise in Pope Benedict's understanding of his mission an expression of their charism of unity, another point that is interesting to note.