The main thing we notice here is all the consonance we have going on with the P sounds. Check it out:
Apparently with no surprise
To any happy Flower
The Frost beheads it at its play—
In accidental power—
The blonde Assassin passes on—
The Sun proceeds unmoved
To measure off another Day
For an Approving God.
Almost every single line has a P sound somewhere in it. Notice how we have double Ps in "Apparently," "happy," and "Approving" (1, 2, 8). We also get some alliteration with "play," "power," "passes," and "proceeds" (3-6). What's the deal with all these Ps? To us, it gives the poem a bit of an edge. It's kind of a percussive, punchy sound, which loads the poem with a little attitude.
We also find it interesting that Emily decided to skip on the Ps only in line 7. Could she just not think of a word with a P in it for that line? Maybe, maybe not. But we like how it gives us a P break (no, not a pee break). The line gets across the image of that uncaring sun cycling through the sky, and not having the aggressive P sound in there allows it to sail smoothly away.
Variations on a Theme
There's more consonance to be found in lines 5 and 6:
The blonde Assassin passes on—
The Sun proceeds unmoved
It's cool how all these Ss give these lines a hissing, sinister sound, which totally works because we're talking about sly assassins and uncaring suns.
Line 5 pulls another sound trick on us by throwing some assonance into the mix. Read it out loud, and you'll hear the repeated "a" and "o" sounds:
The blonde Assassin passes on—
This tightly constructed line slips into the poem like an assassin's blade between a victim's ribs. Whoa, that just got a little dark. Maybe, we've been reading a little too much Dickinson.