While the sonnet had been around for a few hundred years before Spenser wrote Amoretti, he definitely made the form his own. Spenser's big innovation to the 14-line sonnet was a new rhyme scheme. Instead of following the traditional Italian rhyme scheme:
ABBA ABBA CDECDE
Spenser wrote his sonnets with the rhyme scheme:
ABAB BCBC CDCD EE
As you can see, the major innovation of the Spenserian sonnet is the couplet at the end. Those final rhyming lines provide the poem with a major sense of closure. In "Sonnet 75," the final two lines of the poem rhyme beautifully:
Where whenas death shall all the world subdue,
Our love shall live, and later life renew."
With that intense, perfect, rhyming heroic couplet, Spenser underlines the finality of his speaker's word. How can you argue with that rhyme? Spenser was betting that you couldn't.
Oh, and when Shakespeare changed up the traditional sonnet form to fit his needs, he borrowed that couplet from Spenser. That sweet feeling of closure that you get from finishing a Spenser or Shakespeare sonnet is thanks to that little couplet dude at the end.
And, just a note on the poem's metrics, cause we know you guys really dig hearing about meter. The poem is written in almost perfect iambic pentameter. So what does that mean? It means that nearly every line has five ("penta-" = five) iambs in it. Great, so what's an iamb? It's a pair of syllables with the first one unstressed, the second stressed (daDUM). Say "allow" out loud; that's an iamb you'll hear. The only really obvious violation of this iambic meter occurs in the fifth line:
"Vain man," said she, "that dost in vain assay,
The line's first two syllables call our attention, because they are two stressed syllables in a row—what we call a spondee.
This spondee snaps our attention into place, and is like a big red flag: hey, pay attention to what this gal is saying, for crimminey's sake. The poet isn't the only one who's got thoughts and opinions in this poem. The spondee makes the beloved's words stand out.