The poem is indeed one of Tolkien's best -- second only, I think, to the Ring verse. Like the Ring verse, it gives us all kinds of interesting information about Aragorn. In particular, it gives us an interesting look at the manner in which Aragorn and Sauron are related/interconnected to one another.
"
All that is gold does not glitter" This has to be a counterpart, even a counterpoint, to the Ring: also made of gold, and also something that lacks "glitter" in a couple of sensese. I never once remember the Ring as "glittering" which would seem for Tolkien to be a positive idea (the glittering caves of Aglarond). So Aragorn and Sauron are alike in some ways -- both are identified with and by gold, and neither one of them appears to be what he really is. Both are hidden in some way: of course, Sauron is deceptive while Aragorn is cloaked in the protective guise of Strider (interesting that the title of the chapter is this name), but there is this similarity. There is a dissonance between their true nature and their appearance to others.
Of course, the big difference here is that this dissonance is reversed: Sauron, as Frodo points out, seeks to look fair despite being foul (
Macbeth allusion #1 in the book).
"
Not all those who wander are lost;
The old that is strong does not wither,
Deep roots are not reached by the frost." These lines could just as easily be applied to Sauron! After each defeat he is said to "wander" Middle Earth, but he always finds his way back to Mordor where he grows in strength once more. Like the line of the Dunedain, Sauron cannot be wholly quenched or destroyed, only reduced in power for a time.
"
From the ashes a fire shall be woken" The fires of Mount Doom burning once more? The rekindling of Mount Doom in the land of ashes? I do realise that this line is
not alluding to Sauron, but it makes yet another connection between Aragorn and Sauron, this time through the imagery of fire and ash. Again, there's a reversal -- in Mordor, the result of fire is dead and sterile ash; Aragorn, on the other hand, will bring a new fire that will renew the ash. So alike, even interconnected, but opposite.
"
A light from the shadows shall spring;
Renewed shall be blade that was broken,
The crownless again shall be king." I think this is where the conjoined/counterpart/counterpoint relationship of Aragorn and Sauron is most clearly laid out. Aragorn is the light and Sauron is the shadow: but of course, light and shadow are interdependent. In the Christian take on this imagery (and I anticipate
davem here

) shadows are dependent upon light, since the former cannot exist without the latter. It's interesting though that in this verse, the light springs from within a pre-existing shadow (like the fire from the ash). It's almost as though the shadows
do have some form of independent existence and that the light is going to have to combat them. . .
The verse ends with the two items that link Aragorn and Sauron together: the sword and the crown. The sword was broken in the past when Isildur destroyed Sauron and took the Ring, and the crown that is properly Aragorn’s but that Sauron is trying to take for himself.
What I find so compelling about all this is that while it shows on the one hand how Sauron is the shadow of Aragorn, it does so by consistently linking them to each other. Sauron is the ‘negative’ or evil part(ner) of the pair, but they are presented as conjoined, linked to one another by and through the crown (the future), the sword (the past), and even the Ring (the present? Insofar as their conflict is being brought to a head in the War of the Ring?).
The idea of Sauron as Aragorn’s shadow, and the closeness of the relationship as it’s developed by the verse, has a couple of important implications. First, Sauron needs Aragorn, for without the light of Numenor, there can be no shadow of Mordor. Second, though, there cannot be a light that does not cast
some shadows (except, perhaps, the light of Eru?) – so even if Aragorn is able to defeat Sauron, there will always be more shadowy figures cast by his light??
One very last point on the form of this verse, as compared to the Ring Verse: the Ring verse is written in iambs (with the syllables repeating in an unstressed-stressed pattern, like in Shakespeare: “to BE or NOT to BE”; “one RING to RULE them ALL”), whereas Aragorn’s verse is written primarily in anapests (with two unstressed syllables followed by a stressed syllable: “the CROWN-less a-GAIN shall be KING”). Why do I make this point?
*Fordim dons professorly robes and lectures the rapt audience over the rim of his glasses*
Iambic verse is the more usual in poetry as it more clearly mimics the natural cadence of spoken English; it is the least poetic sounding form of poetry. Anapestic verse is the precise opposite; it is the most poetic sounding. This is fascinating to me – Sauron’s verse is one that hides its artifice in a way; it sounds like simple prose, when really it’s poetry. Aragorn’s verse, on the other hand (written, I realise, by Bilbo) is openly artificial and poetic. This really brings the relation of these two characters into focus, I think: both are connected to the same things, and to each other through these things, but one strives to hide behind art (that is, Sauron attempts to look artless?) while Aragorn is more clear and open about his true nature (that is, he is willing to show his poetic nature?).