I was 16 when I first read Keats's sonnet, and it was one of the handful of poems that made me love poetry. It's strange that a 19th-Century poet reading a 17th-Century translation of an ancient Greek poem would give a 20th-Century American boy such a frisson, but I think what thrilled me is the utter daring of the metaphor. Even at 16, I sensed that something subversive was going on: Keats had overthrown the traditional hierarchy of metaphor in which the image—Cortez discovering the Pacific—is subservient to the object it illuminates—Keats discovering Homer. The reader is left with a more vivid image of Cortez staring at the Pacific than of Keats reading Homer, and that is what makes the poem so exciting. The poem underplays Keats's experience and focuses instead on Cortez, and that is precisely what communicates Homer's impact on Keats.
I'd guess that most poets would have chosen to end the poem with images of the awe-inspiring Pacific, but in fact we get very little imagery of either Homer's world or the New World. The poem seems more about the experience of wonder itself than either Homer or Cortez. Keats has chosen to capture the scene at the exact moment when Cortez is looking for the first time at the Pacific but his men have not yet reached the peak and can only surmise what he must be seeing. All our attention is focused on the inner experience.
The subversion of the hierarchy of metaphor seems fundamental to Romantic poetry, because over and over again, the use of something to make a rational statement is eclipsed by wonder at the thing itself. This poem dramatizes how the attempt to use the Pacific to make a point about Chapman's Homer is derailed by wonder at the view itself. Like Whitman, who said that "to die is different from what anyone supposed," the Romantics seem to believe that everything is different from what anyone supposed, once you experience it intuitively. Chapman turns out to be unlike what anyone would expect from a poem or from any book at all, and that is exactly what Romantic poetry intends to be.
In other words, not only is the image of a Pacific a metaphor for Chapman, but the form of the metaphor is itself a metaphor. Our experience of the peak is more vivid and arresting than any comparison of that experience to reading Chapman, and that is the real sign of Chapman's power. Keats recreates his experience for us so that we are left, finally, with a sense of wonder neither at the Pacific nor at Chapman's Homer (neither of which is described in any detail beyond the silence they evoke), but instead wonder at this poem—how it brought us to a place we could never have imagined when we began to read it, and then left us there, "Silent, upon a peak in Darien."