One interesting point about Milton's style is his use of the classical rhetorical elements of ethos and logos while omitting pathos. To understand this, we'll define these terms, then examine one or two instances.
Ethos is the rhetorical technique of building or asserting the credibility of the speaker. Ethos establishes the speaker's or writer's right to speak or write. Milton employs ethos in several places in the opening paragraphs but the clearest example is when he asserts that by speaking out to the governing Houses of Lords and Commons he is proving his "fidelity" and his "loyalty."
For he who freely magnifies what hath been nobly done, and fears not to declare as freely what might be done better, gives ye the best covenant of his fidelity; and that his loyalest affection and his hope waits on your proceedings.
He can assert this because he has previously proven that in a democratic state, built upon the ancient republican freedoms of Greece and Rome, speaking out with valid complaints against the government is the highest demonstration of the liberty they all possess.
when complaints are freely heard, deeply considered and speedily reformed, then is the utmost bound of civil liberty attained that wise men look for.
This leads to a discussion of logos. Logos is the presentation of clear and reasonable logic that proves the case being stated. The above quotation is part of just such a presentation of logos. Milton lays out the logical demonstration that to speak out against the government with just cause to do so is to "promote their country's liberty." He attests that it is with "joy" that he brings a grievance before the government because a nation of liberties cannot hope that there will be "no grievance ever" but that complaints will be "freely heard, deeply considered and speedily reformed."
Now we'll consider pathos. Pathos is the use of heightened emotional appeals to persuade. While some may think Milton's way of addressing the Lords and Commons is a form of pathos employed through flattery--a ploy Milton denounces, "can demonstrate that he flatters not"--since flattery plays upon the listeners' emotional states thus making them more amenable to persuasion, it is incorrect to understand Milton's form of address this way.
it will be attributed first, as is most due, to the strong assistance of God our deliverer, next to your faithful guidance and undaunted wisdom, Lords and Commons of England.
It must be understood that there were strictly prescribed forms of courtly address that showed ample deference and respect coupled with humility (if Milton had foregone these forms of courtly manners, he may have found himself locked in the Tower of London ...). Thus it can be shown that Milton omits the use of pathos from this treatise, which depends upon ethos and logos.