[…] The quick Dreams, The passion-winged Ministers of thought, Who were his flocks (73-75)
As a poet, Keats was similar to a shepherd; he had to tend to his thoughts the way a shepherd tends to his sheep. It's hard work, says Shelley, to write poems.
Our Adonais has drunk poison--oh! (316)
In the poem, poison serves as a symbol of negative and unfair criticism. To unfairly tear down a work of artistic merit is comparable to poisoning an innocent person, says the speaker. That's a pretty serious charge.
What deaf and viperous murderer could crown Life's early cup with such a draught of woe? (317-318)
If criticism is the poison, then the "viperous murderer" is the critic. Shelley didn't mince words here. He thought that unfair criticism killed Keats, and he blamed the critic that wrote it. Don't mess with a poet, apparently.
The inheritors of unfulfill'd renown Rose from their thrones (397-398)
These "inheritors" are dead poets, who apparently occupy a pretty important position in the afterlife. Who wouldn't want to sit on the throne? In Shelley's view, to be a poet is to be royalty, and they should be shown reverence and honor even in the afterlife. We guess poets are shown respect in the afterlife even if they weren't shown any on earth.
And he is gather'd to the kings of thought Who wag'd contention with their time's decay (430-431)
Now Keats has been "gathered" to the place where the other great poets and thinkers reside in the afterlife. He calls them "kings" again, ones who fought against the "decay" of time. That's what makes them worthy of honor, he thinks.