Oooooh! Controversy about a poem! I love this part; I'm about to learn some stuff through the discussion.
The sense I got from the poem was the narrator witnesses something horrific, wants to escape it into the carefree life of a hummingbird, only to have nature/real life/grubbiness intrude into that life. In that sense, the brutality of the dogs killing the opossum and the return to the carcass later emphacize the point.
Now, whether or not I particularly want to have a gritty imagery presented to me is another point. Was it too much in this poem? I think so. Less would have been more, especially balanced against the rather flighty (pun intended) middle section of the joys of being a hummingbird. So I guess a sense of balance is one thing I look for in a poem or story.
As for what's good, some people look for "enjoyable," while others look only for "technically proficient" or "well executed." The first group argues that enjoyability is all that matters, while the others will argue that if something isn't done well it can't be enjoyable. "How can you like Twilight? It's cookie cutter crap." Well, millions did and still do.
For me, it's easier for me to identify when it doesn't work, as with identifying the unbalance of the imagery in the poem. As for well executed, the whole aside of "And whom did I just ask. . ." was meant to be a cute insertion that threw me off. (I also had to double check who/whom. I like
Grammarly's trick: if the answer is "he" or "she," use "who," if the answer is "him" or "her," use "whom." "And whom did I ask? I asked him." It's one of those things I can never remember from one time to the next.)
I'm toying with a list of what I use to determine if something is "good," but the concrete list escapes me at the moment.