What of Milton's characteristics does the speaker admire in "London, 1802?"
I think that one can turn to some of the opening lines of the poem in order to fully grasp where the speaker's admiration of Milton lies. The speaker, presumably Wordsworth, believes that the basic premise of Milton is to provide movement to a society that features resistance to any sort of elevation:
[Milton could] raise us up, return to us again; / And give us manners, virtue, freedom, power.
Consider the idea that Milton is able to empower the current British society to achieve elements such as "manners, virtue, freedom, power." This is what Wordsworth admires in Milton, for he believes that Milton's prose and style can provide something better than what is. For Wordsworth, there is a moral and ethical imagination brought about through art that Milton brought to England in his time and is something that Wordsworth believes can be brought into this current incarnation of England. Wordsworth admires the impact that the poet had on England at one point in time. Through the invocation of Milton now, Wordsworth almost feels that he is trying to do what Milton did. The same qualities of admiring Milton's ability to instill "manners, virtue, freedom, power" are the hopes that Wordsworth himself has through his work now. In this, Wordsworth admires the transformative qualities that Milton possessed making what should be out of what is and this becomes the basis of his admiration throughout the poem.