riding the mountain down into the silver dawn. (20-21)
Check out how the syntax of this line almost makes the mountain seem like it's moving. Sure, if we were to re-phrase this line, it might look something like "my love is riding down the mountain." But, by inverting the order of the object and the verb, Cummings makes the act of riding occur directly beside the mountain. Putting the words closer together makes the action seem more immediate. And that makes the mountain take up more space (in our minds, at least) than the rider riding.
All in green went my love riding on a great horse of gold (1-2)
Right from the start, Cummings plays with the sorts of adjectives that we tend to associate with humans and those we tend to use to describe nature. What color would you use to describe the natural world? Green would probably be high on the list, right? And what about people's riches? In olden times, before the almighty dollar, people carried around gold. But here the green is describing a human, while the gold is used to describe a horse. Hmm… something's not quite what it should be—and we're only at line 2.
the swift sweet deer the red rare deer. (7-8)
Although the natural world sometimes gets described using human qualities, here the deer are described with extraordinary attention. Notice how the first adjective describes their physical characteristics (sweet and red) while the second is more evaluative? We're already getting a sense, even in a description, of some human perception at work.
Four fleet does at a gold valley (19)
Swift-moving deer and a gold valley? That sounds as close to paradise as a hunter could hope to find. No wonder the poem spends most of its time describing the natural world.
the silver dawn (33)
Dawn is the start of day, the time when light is just beginning to shine on the world. It's also a symbol for the end of night, new beginnings, and all sorts of other happy things. Perhaps that's why the death at the end comes as such a shock.