What is Spenser's greatness as a writer?

Two of the multiple things that set Spenser apart as a
great writer among his peers, like Shakespeare and Jonson, and in contemporary times are
his genius for allegory and his uncanny ability to incorporate calendrical (pertaining
to the calendar) and astronomical (pertaining to astronomy) elements in his poetry. A
brilliant example of the first is The Faerie Queene while an
equally brilliant example of the second is Epithalamion.
The Faerie Queene has been well lauded for the detail and
excellence of its allegorical representations. Spenser uses characters, like the Red
Cross Knight--a religious crusader--to uncover and discussion religious and political
truths of his day. He also pays a noble and great tribute to Queen Elizabeth by
incorporating her greatness into his allegory in the character of the Faerie
Queene:

That
greatest Gloriana to him gave,    (20)
That greatest Glorious Queene of Faerie
lond,
To winne him worship, and her grace to have,
Which of all
earthly things he most did
crave;

Epithalamion,
once thought by critics to be unrelated to Spenser's Amoretti
sonnet collection, is now recognized as the continuation, indeed the culmination, of the
great love chronicled in the Amoretti as Spenser sought to win the
love of the younger and higher born Elizabeth Boyle. Epithalamion
is exceptionally great for the way the underlying structure progresses through the
calendar cycles and astronomical cycles. There are 23 stanzas and a final envoy for the
24 hours in a day.

According to Hieatt, the stanzas of 18
lines each correspond to Midsummer's Day, the summer solstice (St. Barnaby's feast day):
"This day the sun is in his chiefest height, / With Barnaby the bright" (470-471). In a
mock lament, Spenser alludes to the winter solstice (St. Lucy's feast day) saying the
longer night is surrounded by the symbols of the death of nature in winter: "'Tis the
year's midnight, ... / Lucy's [with] scarce seven hours ... / The sun is spent" (69-71)
(Edgecombe). There are 365 long lines for the days of a year. There are 68 short lines
for weeks plus months plus seasons of the year (52 + 12 + 4 = 68). When considered with
the amazing structure of the echo refrain with its 17 variations (John B. Lord),
Spenser's Epithalamion is a stellar tribute to his greatness as a
writer and poet.

An additional point is Spenser's mastery
of language. He wrote in a modified imitation of Middle English so that the language of
his works has a much older feeling to it although Spenser was a contemporary of
Shakespeare and Sir Philip Sidney. Spenser also used the smallest components of
language, the individual sounds (the phonemes) to create mood. With a simple change of
dominant sounds, for example from /p, t, d/ to /s, m, n/ sounds, Spenser can calm a
heightening tension, set a scene of soothing tranquility--or by reversing the example,
Spenser can instantly move the reader from tranquility to suspense and tension. In the
quote below from The Faerie Queene, note the change--and its
effect--from dominant /f, l, w/ to a newly dominant /f, g, d,
t/:

Canto
I
XIII
Therefore I read beware. Fly fly (quoth then   
(116)
The fearefull Dwarfe) this is no place for living
men.
XIV
But full of fire and greedy hardiment,
The
youthfull knight could not for ought be staide,
But forth unto the darksome
hole he went,    (121)
And looked in: his glistring armor made
A
litle glooming light, much like a shade,
By which he saw the ugly monster
plaine, ….