How does societal consecration to Mary differ from individual consecration? For individual Catholics, consecration to Mary is a serious commitment not to be lightly entered into. It seems to me that this consecration is seen as an "advanced" step for people who are already accustomed to contemplating Mary, praying the Rosary, etc. For example, Louis de Montfort recommends a three-week programme of prayer before consecration (Treatise on True Devotion to the Blessed Virgin, paragraphs 227-233). Regarding Marian devotion in general, Pius X wrote (Ad diem illum laetissimum, 2 February 1904):


  (17) For to be right and good, worship of the Mother of God ought to spring from the heart; acts of the body have here neither utility nor value if the acts of the soul have no part in them. 
  (20) Whoever moreover wishes, and no one ought not so to wish, that his devotion should be worthy of her and perfect, should go further and strive might and main to imitate her example.


Similar remarks can be found in Paul VI's Marialis cultus (2 February 1974), John Paul II's Redemptoris Mater (25 March 1987), and so on. 

But at the same time, the Popes have frequently consecrated large and diverse groups of people to Mary, including on more than one occasion the entire world. John Paul II said at Fatima (13 May 1982):


  The Mother of Christ calls us, invites us to join with the Church of the living God in the consecration of the world, in this act of confiding by which the world, mankind as a whole, the nations, and each individual person are presented to the Eternal Father with the power of the redemption won by Christ.


and again in Rome (31 December 1984):


  Now is the hour when every person must make an effort to live faithfully this Act of Consecration to Mary.


This necessarily and deliberately includes people who are not Christian, or who are Christians but who disagree with Catholic beliefs and practices about Mary. That is, it looks like this consecration is a first step towards leading people into the Catholic faith, rather than a special additional act by people who are already firmly Catholic.

Is this kind of global consecration in a different category from individual consecration?  What are the intended effects of (repeated) consecration of the world?
My best understanding on the matter:

The word consecration can mean a number of things. In the most basic sense, it's a setting apart of something for a purpose. The significance and meaning of a particular consecration is dictated by the consecrator, the consecrated thing, the consecrated purpose.

In any and all cases of consecration, it is my belief that the consecrating entity holds the primary responsibility for ensuring the consecrated object or person remains properly dedicated to the consecrated purpose. In addition, when a person is the consecrated entity, the person takes on a significant amount of responsibility. In the consecration of a group, the group is also largely responsible; but, only to the extent they were aware of and accepted such a consecration!

For me, it's a toss-up as to wether a self-consecration is more serious and binding than the Church consecrating someone (like a priest). I might lean towards the Church's consecration of a willing individual being the most binding and significant.

And in all cases, consecration "simply" sets the person or object apart to be "used" for a particular purpose. Thus, the consecrating entity is always responsible for overseeing it's "use" or handing off responsibility properly.

In the case of consecrating a large community, including secular individuals, as well as the world, I would understand that to mean, we, the Catholic Church are responsible for guiding the community to serve the intended purpose. I would also posit the effects of reconsecration, can -- but do not always, overwrite previous consecrations.