Especially notable is repetition, but not just by repeating things. The repetitions are a framework for space and place references, which are often metaphors, or for other references, and the repetitions evolve through the speech, and are used to circle back on some of the same themes so they are produced very effectively three or four times.  This is speech is a locomotive, and the repetitions are the track it is barreling down.

You can read the speech here. 

Look for the meter.:

I am happy to join
with you today
in what will go down
as the greatest demonstration
in the history of our nation

There are many,many other segments of this speech that come in a five-five-five or similar poetic meter.

Look for reference to classics/bible:

• Five score years ago, a great American… we’ve come to this hallowed spot. (from Pereclies Funerary Oration and the Gettysburg Address).
• wallow in the valley of despair
• Lots of others

Look for nearly hyperbolic adjectives with repetition:

Fatal to overlook the urgency. Fierce urgency. Urgency of now.

Look for rich metaphors being asked to do a lot of work:

• We’ve come to cash a check, a promissory note written byh the founders, America has defaulted on this note.
• Invigorating autumn of freedom and equality.
• Threshold of palace of justice
• Cup of bitterness and hatred
• Winds of police brutality

Look for building metaphors on metaphors:

America has defaulted on the check but the bank of justice is not bankrupt.

Look for poetic repetition:

• One hundred years later…
• We cannot be satisfied
• I have a dream dream dream
• Let freedom ring

Look for metaphor mixed with repetition and indirect reference, esp. using space/place:

Cannot walk alone
As we walk… we shall always march
We cannot turn back.

Further look at the “I have a dream” part

• “I have a dream that” = five beat rhythm
• “I have a dream that” repeated four times, then shifted: “I have a dream TODAY!” Twice
• Many of the “I have a dream” clauses re-visit earlier parts of the speech, stating the same idea again but in this elevated prose.

Look for use of place and space, including spatial shifts in phrases, and where the adjectives sit in the language:

In Dream section, place (and person) repeated: This nation, just Georgia, just Mississippi, Alabama, Every valley, every hill and mountain, etc.

Mountain of despair, Every valley and every hill and mountain, rough spaces made plain, crooked places made straight, etc. etc.

“Let freedom ring” repetition, repeating themes for THIRD time, making use of space and place AGAIN. by this time the listener is totally in the groove with respect to the framework of metaphors, the cadence of repetition, and the space/place framework.

Again, five beat repetition in “Let freedom ring from”

Rich juicy adjectives for each of the places mentioned (mighty, prodigious, curvaceous) replaced for a few beats with a built in strong adjective as part of the place name (Stone Mountain, Lookout Mountain) etc.

Reverse of ring repeat in first “last line. “

These last two things (have a name in rhetoric, I forgot it if I ever really knew it) bump the listener.

Denouement

Full of repeats, classic references, more place/space references, the whole shebang in one little paragraph followed by:

Powerful repeats (“free at last”) with a couplet of iambic pentameter to finish it off:

Thank god Almighty
We are free at last